Honestly I think that's too young.
It varies by kid, and you know your kid best, but 3.5 just seems way too young to me. My kid was 11 when we took him to his first concert.
Your son will have to deal with loud noise (noise cancelling headphones are a must for a young kid at a show, not just earplugs), obnoxious other concert goers who are drunk, swearing (and singer who may swear) and possibly smoking weed nearby. Obviously a U2 crowd is tamer than some other crowds, but there are still plenty of buzzed and drunk folks in the audience.
And he'll be up presumably well past his bed time, are you prepared to leave early if he can't handle things?
Just my $.02, good luck with what you decide to do.