On Sunday afternoon, Alex Mann was an ordinary teenage rap fan from England’s smallest city, enjoying himself at his first Glastonbury festival. By Monday morning, he had been plucked from the crowd at the Other Stage to perform flawlessly in front of thousands – and turned into a social media star.

The 15-year-old in a bucket hat from Wells, Somerset, set social media alight with his barnstorming appearance alongside Dave as the pair performed the rapper’s track with AJ Tracey, Thiago Silva – named in honour of the Brazil and Paris Saint-Germain football player. It was, Alex told the Guardian, “the best time of his life”.

In a widely shared BBC video, Dave – full name David Orobosa Omoregie – asked the crowd: “Who is sober enough to sing these lyrics with me?”

Alex, sitting on the shoulders of a friend, was wearing a PSG shirt with Silva’s name on the back, and pulled at the club’s emblem as his companions pointed at him, trying to get Dave’s attention.

Dave, who spotted Alex gesticulating, said: “I see a PSG shirt over there, but do you know the lyrics?” After Alex responded with a few confident lines, Dave concluded: “He looks like he knows the lyrics … Yeah, let’s take a chance.”

As Alex took to the stage, Dave briefly reassured him he would only need to cover AJ Tracey’s part – but instead, Alex rattled through every single word of the complex, quickfire tune without hesitation, drawing an admiring pat on the back from the slightly upstaged Dave, and the adulation of the crowd.

Alex Mann demonstrates his skills before joining Dave on the Other stage at Glastonbury. Photograph: BBC

On Monday, the Guardian contacted Alex via his mother, who said that he would be able to speak after he had had a sleep. When fully rested, the teenager said he had no hesitation about heading to the stage after Dave called on him. “I thought he might be joking so I thought I’d better get up as quickly as possible,” he said.

He added: “I’m not sure what was going through my head. I was so nervous. I thought: ‘What if I mess up?’ But then I went on stage and performed and it felt so good.”

By Monday Alex had received a message from Thiago Silva. The footballer, currently preparing for Brazil’s Copa America semi-final against Argentina on Wednesday, congratulated Alex on his performance, and asked for his number to speak over the phone.

“I haven’t spoken to him yet, but we’re in different time zones,” Alex said. “I’m not really sure what I will say to him. I really like him as a player, so I’ll probably just say that.”

Dave, a rapper, singer, musician and songwriter from south London, whose rich, personal lyrics have won him a string of hits and an Ivor Novello award, has also been in touch.

“He was supportive and said that I would get a lot of messages today and if I needed anything to let him know. He said if it got overwhelming he would be there,” Alex said.

He added: “When I was on stage, he [Dave] looked at me and said that if I got stuck and didn’t know the lyrics I could look him in the eyes and he would help.”

“I’ve listened to the song plenty of times, to be fair, but I was worried when I went on stage. I didn’t think I would know all the lyrics but I did … I pick up lyrics quite fast,” he said.

After the performance, Alex was stunned by the reaction. “I walked off stage and the security guards took me back down and it was just amazing,” he said. “The way security brought me back down was in front of everyone and I was walking past people who were screaming my name. I had the best time of my life.”

He admits the attention has been a lot to take in – but so far Alex is taking it in his stride. “Lots of people have started following me on Twitter and Instagram and liking all my posts … I am not going to let it get to my head too much.”

It was the teenager’s first time at the festival and he was there with about 20 friends. “I’ve not been before but my mates said it was one of the best Glastonburys so far. It was so hot and there was a really good atmosphere, no one being aggressive. It was just overall good vibes.”

Some sceptics online have dismissed the video as a stunt, believing that no amateur teenager fan could have performed so precisely. But Alex insisted that there was no such set-up. “It was 100% real,” he said. “Completely just happened … I have only ever seen him once before at a show in Bristol.”

For all the ‘it’s a set-up’ bores, Dave regularly asks for someone in the audience to rap the AJ Tracey bits in Thiago Silva.This kid knew that cos he’s clearly a Dave fan, so he put on his TS shirt, got down the front and hoped he’d be picked. Dave saw him and he smashed it, end https://t.co/z7YEyM3aEw — Miranda Sawyer (@msmirandasawyer) July 1, 2019

He also sought to correct social media claims that he had not been completely sober for the performance. “It’s funny reading the comments,” he said. “If I was drunk I wouldn’t have been able to remember the lyrics like that.”

As for the future, Alex said he had never considered trying to turn his performance skills into a career. Instead, he said, he was “just focused on getting it right” on the day. Still, the reaction from the crowd made him realise he had some talent. “I’d be up for performing with him again,” he said. “And being in a video.”