Sorry kiddiepshrink but I couldn't disagree more. My oldest son is now 15 and despite my having babies when he was 6, 8 & 11 he NEVER asked a single question about where the come from, how they get there, how they come out, or anything. And I was ready and waiting for the opening, I swear! I researched answers, read books, and figured that at some point, a question would come up and we'd be off and running. Nope. Never, ever happened.
I know that my oldest son has had a comprehensive education on all of this in school. I only know this because my friends who have kids in his grade talk about what their kids thought of this class or that movie or whatever. I started bringing some of this up when I know that he was having these classes that talked about puberty, development and later sex, protection, etc. and he would quickly shut down the conversation and tell me that he knows everything. We were usually in the car so I would say what I thought was important with him yelling "OMG mom stop!" So...I know that I am the last person on earth that he will come to with questions or concerns or seeking information. I just hope that what he learned in school has sunk in and that his girlfriends have brains.
With my younger sons, it's been a different story completely. They were just curious and are not shy. They were 3 & 5 when I was last pregnant, as a surrogate. My 5-year-old (who is now 9) wanted to know everything. So he learned about uterine cancer, what a uterus is, what ovaries are, egg harvesting, egg fertilization, embryo transfer, ultrasounds, and how babies are born - as a Kindergartner! My youngest, who is now 7, made up a song that goes "van-gina, van-gina, if you have one you're...a girl..." and they are not at all squeamish about asking questions.
If I were you, I would hit the library or book store and see if there's a book you like on development in general. I know that for girls, there is the American Girls series - I don't know if there's something similar for boys. Then just casually introduce the topic. Maybe just go ahead and ask him something like "have you ever wondered where babies come from?" and take it from there. I really wish that I had stopped waiting for my oldest son to ask and had just gone ahead and started up this subject when he was younger.