I might not have any kids and I'm only 26, but let me take a stab at this.
I think at thirteen he might be going a little fast. If he's already started advise him to slow down a bit life goes by fast enough. Find about his friends, girlfriends if there are any. See if you can figure out what's going on in his head. What triggered this? I would guess he's got his eye on someone and isn't sure how to move forward. Exploring his own sexual urges might just be his way of dealing with it. If he is considering someone give him a little push, help him talk to her, see what she likes and try to get into the same things (so long as it's not hair and makeup of course). I honestly think there's an individual there he's not talking about and isn't relating with. Does he get into the Z's with you? If so working on one of them together might be a good time to strike up the conversation. Hope this helps.