Damn, that's a terrible situation to be in. I think you should start of by reinforcing your love for your son, before attempting any sort of discussion on the subject. You could also try to expose him to other world views, without bringing up the subject at all. The age difference might be a factor here; but when my 5 year old kept talking about God (Because her fuckhead teacher decided to try to indoctrinate her into christianity), I started telling her about things like wicca, islam, scientology etc. I then asked her which of those ideologies sounded good to her. She is a Harry Potter fan, so she chose wicca. I went on to explain how sometimes people choose a religious standpoint solely because a particular aspect of it sounds good to them, or it's the only one they've ever been exposed to - and after the fact, they will accept everything else that it teaches. As I said, this worked with a 5 year old but I'm not sure it will with a 13 year old.
Good luck with it. As a father, I do not envy your position at all.
Good luck with it. As a father, I do not envy your position at all.