My son wants to get laid

[n.s.]

If he's feeling controlled and frustrated, that means that he still cares about the rules that you set out for him, which is a good thing. When he starts acting nonchalant about the whole thing is when you'll know that they've found a way to go behind your back.

Take advantage of this and level with him now, before he completely stops caring about your rules. It's not your responsibility to enforce her parent's rules - if the girl wants to disobey her folks, that her choice (and your son's, somewhat), not yours.

Talk to him honestly about this stuff, but for god's sake let him close the door. Maybe they'll get freaky, maybe they'll just smooch, or maybe they just want to have a private conversation. In 2 years this kid will be a legal adult and will be completely out of your control. It's time to trust him with a little bit of privacy in his own room.