Alfie, 10, was attending his first gig with his mum and dad, Lisa and Kevin, all from Luton.
He got the chance to meet Grennan backstage, describing it as an “absolutely incredible” experience.
Asked by the singer what he was looking forward to he agreed it was the “energy” of the gig.
The two even also discussed England’s chances at the World Cup.
Alfie told Grennan “I’m not very confident with Mexico,” with Grennan agreeing.
The singer added: “But you know what, whatever happens, we’ll still scream and we’ll still shout won’t we?”
Grennan also told the youngster he had never been a Luton Town supporter despite having previously played for them, saying he was actually a Coventry City fan.