To get back to the OP about whether your son should go or not, I would be cautious. Cautious? At a Christian event? Why?
If you son has just now come to believe in God, he is still, shall we say, impressionable. Having said that, I have been to Christian concerts in the past. First and foremost, let me say that the music is AMAZING. To hear God’s name glorified in such a way is inspiring. I truly did enjoy every concert that I went to. The thing that you want to be aware of is that, while these bands are Christian, they are not Catholic. Sometimes at the concerts they will do “alter calls” where you profess your devotion to Jesus Christ and become “saved” as they say. Mind you, this is not what we as Catholics believe or how we live our lives. With your son being so impressionable, it would lead me to believe that being in this place, with the inspiring songs, feeling a ton of things that he has not felt before, he may “buy in” to this and think that this is the way to go and this is what religion is all about. If you want to raise him in a Catholic home, just be aware that Catholicism will not be stressed at this event.
Not that the concert would be bad, I am not saying that at all, I think that Christian concerts are a wonderful thing, but again, you have to be knowledgable about your faith so that you are not sold something that you do not truly believe to be true (i.e. being “saved”)