Knowing a little of your own religious beliefs, you could either laugh it off with your son and explain that she has got it wrong and that you are in fact a true christian, in which case why would you go to hell or give him my views of it, in that case no heaven, no hell, life is great :-) after all, I made my mind up and decided on atheism also at the age of 12, just guide him to make the right decision, give him your very honest views. Help him make an informed decision.