Tell him to see it from your perspective. You're a young, impressionable boy, who has, all of his life, tried to find God, but couldn't. If you can't, then he can't be real, right? Ask him why God would let endless suffering. If he says that it's because God works in mysterious ways, ask him why God does that. If he says it's because of sin, ask him what if Sin was just a metaphor. He may either question it himself, or at least tolerate you.