Okay- well it honestly depends on how MUCH you want to tell him. He might be a little more comfortable talking about it with his dad. My mother gave me a LITTLE information and then handed me this book called 'WHERE DID I COME FROM?' Find it at Amazon here: http://www.amazon.com/Where-Did-Come-Peter-Mayle/dp/08184...
This was a great book, BUT considering I grew up in a conservative Christian Family and raise my own two the same way it may seem pretty graphic with the Cartoon illustrations. I used that book and blended it to discuss sex and the importance of waiting until marriage to present that as the most wonderful gift that spouses can give to each other. I will spare the details here, but my now 13 year old comes to me for the answers of some of the school yard talk- I would rather her find out from me, than to find out the hard way later! Thankfully I go to a WONDERFUL Church (Lake Pointe) in Rockwall and they often have classes that discuss great way to talk to your children about it. SO- a long answer to your question- but I am not sure what angle (if any) that you would want to give your son when talking about a topic that our parents thought was taboo to discuss.