The last time Lewis Capaldi visited Times Square in New York, he was confronted by his own face: a poster for Spotify stretching the height of a building. It featured him wearing a selection from his apparently inexhaustible collection of idiotic-looking sunglasses: a white-framed women’s pair, not unlike the ones Kurt Cobain took to wearing near the end of his life. He looked at least a little more like a rock star than he usually does in promotional photos. A British poster campaign features what is supposed to be his Tinder profile: a photo of Capaldi grinning gormlessly, his hair wrapped in a towel. He is flashing a peace sign and the caption reads: “Otherwise known as the Scottish Beyoncé.”

Of course, he took a photo of the Times Square poster and stuck it on Instagram and Facebook. In a perfect example of Capaldi’s approach to social media, he listed his location as “GENERAL ERECTION” and counselled “nyc pls don’t trip over yr boners xox”. The thing is, he asks, as he demolishes a burger in a Chicago restaurant, how else are you supposed to react? These ridiculous things keep happening to him.

He posted a video of himself frowning while listening to Camila Cabello’s new single, mock-horrified that it could outsell his single Someone You Loved in the US. The next thing he knew, Cabello had covered Someone You Loved, “and obviously she smashed it, doing all these vocal runs I can’t do – I was like: ‘Can’t you just sing it without showing me up?’” The other night, there was a knock on his dressing room door and David Hasselhoff was standing outside: when Calvin Harris commented on the ensuing Instagram selfie, Hasselhoff immediately nipped in and suggested the three of them make a record together, “which would obviously be fucking excellent”, Capaldi laughs. “Jump in My Car, but with Calvin Harris producing. The last time I saw David Hasselhoff in the flesh, he was playing Captain Hook in the panto at the Glasgow Armadillo. I was there with my family the day after Christmas, pished. And I’ll tell you something: his performance was a revelation.”

It’s all so weird, he says, that you have to laugh at it. “You cannot fathom some of the stuff when it is going on. It’s just daft. I sometimes worry that people think I’m just talking shite, and feel I’m not as surprised with things: ‘Oh, he’s been doing so well now that he must be used to it.’ But I’m not. It’s fucking wild.”

It is indeed. A year ago, Capaldi looked suspiciously like he could be a one-hit wonder. One of a glut of earnest, dressed-down acoustic singer-songwriters the Guardian subsequently referred to as “ordinary boys”, he had been plucked from a life of obscurity playing pubs around his home town of Bathgate, West Lothian, by a manager who stumbled across the songs Capaldi was uploading to SoundCloud. “My immediate reaction,” notes Capaldi, “was: ‘Fuck off.’” His single Bruises became a vast streaming hit – 28m plays on Spotify, apparently unprecedented for an unsigned artist – but its follow-ups did nothing like as well. “I shat myself,” says Capaldi, “because I thought the single we’d released – Tough – was the best song I had, and it hadn’t done anything.”

Someone You Loved.

It wasn’t until Capaldi’s label released Someone You Loved that things went ballistic: seven weeks at No 1, the biggest-selling single of the year in Britain, and a platinum hit across Europe and Australia. His album, Divinely Uninspired to a Hellish Extent, is the fastest-selling debut by a British artist in eight years. Then it started selling in the US.

The day I meet him, Someone You Loved is No 3 in the US chart, causing Capaldi to start referring to himself ironically as “America’s sweetheart”, posing for photographs in an Uncle Sam outfit while clutching a bunch of roses and plastering the slogan all over his US tour merchandise. “It started as a shite joke,” he says. “Because I’m not America’s sweetheart, do you know what I mean? I’ve got one song. The album has only just started to move here. But then people who come to the gigs are making it a thing. They started screaming: ‘America’s sweetheart!’ at me. They’re making little videos about it online, so it just became: ‘Well, how stupid can we make it?’ So we rented an Uncle Sam costume, really taking the piss.”

The music should be serious, and everything else should be fun because it is fun

“Of course,” he deadpans, “I looked absolutely tremendous in it.”

He seems baffled as to why it has happened; why he – out of the serried ranks of post-Ed Sheeran singer-songwriters – became the one who is selling millions and the first artist in history to sell out a UK arena tour before he had even released an album. “I’ve got no clue. ‘What right do I have to be playing arenas?’ is what goes through my head. Impostor syndrome. I’m not better than any of these people, I’m not better at writing songs or singing.”

The prosaic answer is that none of his peers had a song that captured audiences’ imaginations the way Someone You Loved has. It is a breakup track that acts as a kind of evil twin to Adele’s Someone Like You; that song’s stoic resolve replaced by wounded disbelief. It might also have something to do with Capaldi’s voice – a full-throated, emotive roar that he alighted on after hearing Joe Cocker’s 1969 cover of the Beatles’ With a Little Help from My Friends as a teenager.

‘Delusions of grandeur can be very, very dangerous indeed …’ Lewis Capaldi on stage at Thalia Hall Photograph: Michelle Kanaar/The Guardian

“I could always sing loud because I was playing pubs when I was 11, and you’ve got to sing loud there. But I saw Joe Cocker on some Beatles documentary, and I was floored. I tried to sing like that, and I was fucking terrible, as you would expect from someone going through puberty trying to give it the raspy voice,” he laughs. “But then I finally fell into it at around 18. I don’t know if I’ve done some irreparable damage to my voice trying to get here. I never had any lessons, but obviously now I’ve had coaching to keep it going. I get really bad acid reflux, so I’ve got to watch myself on tour: no spicy foods, as little alcohol as possible.” He sighs. “See Mick Jagger, Elton John, all these cunts on tour in the 70s, going around doing everything, taking all these drugs, sounding great. I take one drink of Red Bull and I’m choked.”

But there’s something else about Capaldi’s fame. He is incredibly likable in person: funny, warm, and implausibly fond of swearing (looking at the transcription of our interview, he said “fuck” or “fucking” 243 times in the space of 90 minutes). Moreover, he has proved very adept at projecting that likability on stage – where what he refers to as his “blether” between songs occasionally lasts as long as the songs themselves – and on social media.

His Instagram and Twitter feeds are unfailingly hilarious and self-deprecating. In a pop world filled with artists telling audiences to be themselves while posting perfectly filtered selfies, Capaldi is himself, whether he is mocking his chubby physique (“It’s an honour to have been #blessed with these abs & to finally be able to give back and help others reach those #jacked goals is beyond a dream”), parrying the insults hurled his way by Noel Gallagher or talking about his own mental health issues. He started having panic attacks on stage a year ago, became convinced he had a brain tumour and paid for an MRI scan before learning to quell them with cognitive behavioural therapy. He plans to set aside a space at his gigs so that other people with anxiety issues can attend in relative comfort.

US fans queuing for Capaldi’s Chicago gig. Photograph: Michelle Kanaar/The Guardian

He is a very modern kind of pop star: part singer-songwriter, part social-media celebrity who easily #relatable in an era when #relatability is a hugely saleable commodity. He says his record label has never suggested he employ a stylist or go to the gym. And he calls me out when I refer to his social media campaign. “You say ‘campaign’, but no one’s told me to say anything. It’s just me, jetlagged in a room somewhere, talking absolute shite. The self-deprecating thing – that’s just how people from my home town talk. I love the comedian Limmy and the way he pushes things until they get more and more stupid. It always makes it funnier when everyone else has stopped laughing and you’re still, like: ‘No, this is brilliant.’ The music should be serious, and everything else should be fun because it is fun.”

Besides, he says, you may as well enjoy it because you’ve got no idea how long it’s going to last. He’d be happy, he adds, if his second album does even half the sales of the first. “Maybe it’s the way I’m wired; maybe my mum and dad have put pessimism in my veins. But I think it’s a good way to be. Delusions of grandeur can be very, very dangerous indeed. You have to have a word with yourself, like: shut your fucking hole, you’re living your dream here. Going: ‘Hi, I’m Lewis Capaldi and you’re listening to Chicago’s B96,’ – these people are playing your song, so just get on with it. My pal at home, he literally digs graves for a living. So saying: ‘You’re listening to my new song,’ on some radio station, it’s not really a hard time.”

And off he goes to get on with it. That night, Thalia Hall in Chicago is sold out: the queue starts snaking around the block hours before showtime. The audience know every word of every song, and do seem to be taking the idea of Capaldi as America’s sweetheart seriously, amending chants of “USA!” to: “Lew-is-A!” Just before the closing performance of Someone You Loved, persons unknown throw a pair of stars and stripes underpants on to the stage. He pulls them on over his jeans. “I hope these are XXL size,” he nods. “Because I have a massive penis.” Then they start chanting again.