Set him down and try not to be real serious about it,( frowning on it and the likes). Let him know the truth, it's fun, it's really fun, and with the right girl it's a whole lot of fun.
Ask him what he want to be when he grows up. What are his hobbies? Explain to him that sex is okay but he needs to make sure he is doing it safely, 1 kid with the wrong girl can keep him from having everything he wants in life.
Let him know there is nothing wrong with it but he needs to be careful.
And if a girl tells him she wants a baby, and it would make her happy, let him know he doesn't have to be the one to make her happy, and he should run like hell.
Just tell him to take his time, don't rush into it, and don't let your friends know what you did with so and so, cause it will make him look like an ass, and no girl is going to want to do it with him if he's running at the mouth.
1 be respectful,let him know the benefits to being respectful to the girl.
Most of all from a parents view don't be too uptight about it and talk to him like a friend explain it to him like an older brother. Dont be too overbearing our you might regret it later, he'll do the wrong thing just to spite you.