Lisamarie, I think getting a book on the subject is a great idea! Offer to read it to him (if he can't read yet) and don't push it if he's not interested, just keep it around for when he's ready. It's good to have the book so he knows there are ways to get this info other than asking questions, which he may find embarrassing.
Mama to a boy EnviroKid
10 years old and a little girl EnviroBaby
I write about parenting, environment, cooking, and more.