A friends DS pointed and laughed at Nina (when they were in the bath) one time and asked 'what's that mummy'! He was 18 months at the time. It was actually quite funny the way he did it. As he is showing interest, tell him, and tell him the real words for all the areas. You don't need to go into detail about it at his age, but just say 'that's mummy's vagina' or something like that.