You can no longer micromanage your son's activity
You have probably noticed that your can't control your son's actions by force or persuasion anymore, if you ever could. He's old enough now to be self directed, which is scary. The only thing you can do is to give him a moral framework in which to live.
Provide a credible moral framework
Can you explain to yourself why you think this is wrong? Is it purely because he could get into trouble with the law? Is it because of your religious convictions? Is it because you're worried he or his girlfriend might get hurt? Teenagers typically have a fairly high tolerance for risk and warning him about heartache or legal consequences is unlikely to have an effect.
Give him the information he needs to make the right choice, then trust him
You need to show him how to act, but more importantly explain why he should act in that way. You can't prevent him from acting by force or threats any more. He will make his own decisions now and there's little you can do to prevent that, so give him the information he needs to make the right decisionschoice.
After that all you can do is trust him. That's a pretty scary thing.
Best of luck.