The 'Someone You Loved' hitmaker admits he sometimes questions why he is the one selling out arenas because he's "no better" than other artists.
He said: "I've got no clue. ‘What right do I have to be playing arenas?’ is what goes through my head. Imposter syndrome. I’m not better than any of these people, I’m not better at writing songs or singing."
And the 23-year-old singer - who hails from Glasgow - has been singing since he was 11 but didn't "fall" into his voice until he was 18.
Speaking to The Guardian newspaper, he added: "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 f***ing terrible, as you would expect from someone going through puberty trying to give it the raspy voice. 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. See Mick Jagger, Elton John, all these ***** 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."