I would have to agree with Greg's (spamolds) advice. My son converted at eighteen and I think one thing that put us off was that we felt we were being judged or that we were inadequate. He was very much "full of the fire" and to be honest, very overzealous.
He has since toned down and leads his life by example. I see the good it has done for him and am going to church with him most sundays now.
When I ask him questions, I make sure he knows I'm not attacking him. His answers have gone from being argumentative, to kind, reflective and understanding of our lack of knowlege and understanding of his world.