I just sat down with mine and talked to him about it. He's always known anatomy differences since he was like 6 so he was fully aware of the full physical differences between men and women.
I just asked him if he knew anything about the process of babies being made and sex, he said he heard a few things and then I let him ask whatever questions he has while being as thorough as possible and filling in whatever gaps he had. He drank a Coke, I drank a beer and we talked for like an hour. The biggest thing was keeping it casual so he didn't feel embarrassed by asking questions. Because of that he's come to me to ask more since we had the talk since he knows he can without feeling weird about it.