Chances are he already knows a lot about it.
Kids are finding out at younger and younger ages about sex.
First, you have to find out what he THINKS he knows (a lot of the peer information is also incorrect). Then, you have to make sure you are open and honest, letting him know he can always come to you with questions. Most importantly, you must remain calm.
Otherwise, you may not be able to build a form of open communication between the two of you.
Most children and parents have certain ways of talking to each other, through jokes or activities, maybe you are comfortable enough to go in his room and speak to him outright about it. That's something you'll have to decide. I've included two links with further information.
The first is an article on the subject.
The second link is a site entirely devoted to talking to kids about difficult subjects.