You have the "talk" when they start to ask. I don't believe a child is ever too young. We are open in our house, and it has proven to be what is best for us. Honesty and openness help you protect them. You aren't quite at a place of him being sexually active, but the time will come eventually. Better for him to learn from you then his peers. That happens NOW! Kids are much different then in the past.
When it comes to later (teenage years), I have learned that condoning it isn't good, but making sure they are safe is better. There can be a balance. I prefer they don't do it at all. Ashley is now 18 and has only had one partner. She wishes she hadn't now, but due to our willing to talk with her, she made the decision to wait longer. Her brother is 16 and we believe he has finally done something with his girlfriend of 5 years. We aren't sure, but are pretty sure. Our 14 year old is a virgin, and she wants to stay that way.
Please, don't think I am saying I think it's okay. But I am so honored the children are comfortable talking with us. Better for them to come to us and say, what does this mean, how does that work, should I or shouldn't I... If they ask their friends, lord knows what they would say.
In saying all this, I believe if your son is asking, tell him.