J.V.
If this is what he ended up asking you, imagine what he already "thinks" he knows. If he is getting information on sex, I think it needs to come from you and not from his friends at school. Once that information has been given to him, it's harder for you to correct it. You may need to use a book as a guide but keep everything very age appropriate.
My daughter has been given the 3 year old version of this talk and the 4 year old version. She know's very "high-level" information. If she asks me in more detail I will only tell her what I think is appropriate. Basically, I tell her things that I think will keep her safe and that will let her know when she is being touched inappropriately. So I believe the sex talk can begin on many levels at any age but the earlier the better and like a house, you start with a good foundation and go from there.