do not tell him the baby comes out your butt! my little brother found our cat giving birth and my mom told him the kittens were coming out of her butt because she didn't want to explain what a vagins is and he walked up to anyone who would listen and told them how he saw our cat pooping out babies. avitoria had a very good answer. Only tell him what he can handle at his level of maturity and don't base that on his age. There were things my mom could tell me before I ever started school that she still can't tell my younger brother and he's nearly grown. If he has a habit of talking about bady parts in a nasty way, I wouldn't go into too much detail. If he handles those things fairly well, you could probably tell him a little more. Just remember to never lie to him or refuse to tell him things. It'll close him off to you so that he feels he can't tell you things when he's older.