I would honestly say 8-12. My reasons is that at that age, its something they could get used to quickly and its not in the teens where it will bite ya in the butt, every time they get mad at you. I'm sure it might be said here and there. But if this is something you don't want to hide and want him to be comfortable with growing up I'd say then. Old enough to understand, but not too old to resent you from holding it from him either. what to say, ummm, maybe something along the lines of, that she was scared of the responcibility and didn't think she could give him the best life possible, and wanted to do the best thing for him that she knew how.
I don't really know, you don't want him to really hate her, and you don't want him to ever think he wasn't wanted by someone, especially his bio mom. That could really hurt his little heart. *hugs* this is a hard one.