What kinds of questions is your son actually asking? I think I'd probably also avoid the book with the "Is my penis too small?" question but we do have several children's encyclopedias with diagrams of reproductive organs (and one drawing that shows a cross-section of the organs of both sexes joined in coitus!) Just personal preference, I'd prefer DD's info from a book to be more clinical and anything further to be from her parents.
I agree you should find out the "why" of any question on the subject to make sure you're answering just what he's asking.
Here's a convo that took place between me and DD6 in the car this week:
DD6: How does the baby get inside the mom?
Me: (brief explanation of eggs, sperm, cell division)
DD6: Hmm. But how does the Daddy get the sperm to the Mommy's egg?
Me: Well... I can show you in your children's encyclopedia when we get home.
DD6: I bet you have to get REAL close!
Me: ...uh-huh... but just getting close to somebody doesn't mean you're going to make a baby with them; there's more to it than that.
DD6: Oh, so like kissing?
Me: Um... you could kiss but just kissing doesn't make babies- there's definitely more to it. Do you want me to show you the pages on reproduction in your encyclopedia when we get home?
DD6: Yeah.
(Later, we're home)
Me: So do you want to run up and get your encyclopedia then?
DD6: Nah, I'll save that for later.
Me: *whew!*
