D.T.
Asking him what he knows is a good start. After that I would stick with very simplified facts, but talk to him like an adult.
An example: "Sex is something very personal and special and private between two people(or between a man and a woman after they are married). When two people (or the couple) love and care for each other and want to show it in a very special way(or when they have decided to have a baby) they have sex. It involves a lot of hugging and kissing and a few other things you don't need to worry about right now."
You can be as liberal or as conservative about it as you feel comfortable. I showed both options. Hope it makes sense.
Good luck.