When my dad said, "It's time we had a talk about sex." I said, "OK, whaddaa wanna know?"
He told me that he's sure that I've probably been told some wrong things and I proceeded to give him a very accurate account of how humans procreate. I was a geek so I had studied it from a medical standpoint. He goes, cool, I did learn something, you'll be fine!!
I have a 10 year old son. He's starting puberty so I asked him if he knew what was going on, why certain "parts" have their own mind, etc. I got detailed enough until he said, "Stop, I think that's enough for now."
We've slowly added every few months as he's ready for it. It was actually quite good for him since he had started wondering if something was "wrong" with him.
I'm sure your son is past that point, but keep that in mind. He probably does know a lot. I say be real upfront and frank about it. A little humor (just a little), helps, too. Chances are he has some questions and as long as you don't lie and don't hide something because you're uncomfortable, you'll be fine. It's critical to be honest at this point. Anything you hide or lie about (because chances are he knows the answer and is getting confirmation), might make him weary of asking again.
Good luck. You WANT to talk to him about it. You WANT to do the right thing. You're already more successful than 90% of the parents out there.
-Rob