In the belly of Scotland’s biggest ever indoor gig, they’re knocking them back and belting them out for one man and his guitar on a distant stage.

Heads tilted up, chests puffed outwards, arms thrown skywards and pints spilled everywhere, the crowd sing along – actually, bellow – to foot-stomping, Glasgow-accented, locally-slanged songs about chancers who ​“dance like a dafty for a bag of snow”. About a guy who’s ​“a lightweight, one or two jars and he’s buckled”. About a girl who’s ​“a gangster with a hundred-mile stare/​When she walks, her feet don’t touch the flair”.

It’s a freezing, rain-sodden Saturday night in the recently opened P&J Live arena near Aberdeen Airport – a location that makes sense given that this eyeball-bogglingly wide concert hall has the dimensions of an aircraft hangar. When all the rows of seats are rolled into wall recesses, its 9000 square metres give it the biggest arena floorspace in the UK.

Tonight the concert promoters need all that floorspace to accommodate the 15,000 fans – all ages, all sexes, couples and gangs, kids and parents, teenagers and more teenagers – who’ve travelled from all over Scotland for a show that sold out in a couple of hours. Liam Gallagher played here last week; Rod Stewart’s coming in next month. Neither, and no one, can sell as many tickets as this guy. Tonight’s show is the final night of a UK and Ireland tour that ran through November and which played to 125,000 fans.

With most tickets costing 30 quid a pop, the gross for this single show is something of the order of £450,000. Then there’s the merchandise income. That’s for a guy who, a bass player hidden in the wings notwithstanding, performs alone. Just him, some acoustic guitars, a harmonica and foot pedals that trigger a bass or kick drum – plus, to be fair, some impressive, not-cheap confetti-cannons and pyrotechnics. Of course, the live artist takes home nothing like all of that gig dough. But still: even if he has a serious plectrum habit – and if his preferred Adidas trackie tops, kagoules and trainers are costing him (they’re not) – that’s a tidy, well-earned profit margin.

What else? His robustly-melodic, singalong Scotpop-folk anthems about drugs, mugs, thugs and ordinary life have 100 million cumulative Spotify plays across his Top Ten most-played tracks. His debut album has sold 132,000 copies – and that’s an album he wrote, recorded and paid for himself. He’s an unsigned artist who, this month, sold out Hampden Park, Scotland’s national football stadium, in less than a day. That’s 50,000 tickets, gone, in a few hours. He could have sold out two.