Boy, does this all sound way too familiar. I'm only 22, so not too long ago was I 17 and had met my now husband. I never snuck out, but i lied to my parents plenty of times and have been caught. No one ever gave me the sex talk in my family, I had to learn on my own.
My now Mother in law once called my brother in law's girlfriends' parents and told them how slutish their daughter was acting and that her son didnt need that sort of distraction. My brother in law has NEVER forgiven his mom for this and has not dated again since. That was 4 years ago and he is now 21...still not interested in any girl but Becky. But Becky has moved on...he hasnt.
In your situation, with having a son (not a daughter) I would ask him to be honest with you and if you already know he's having sex, tell him you dont agree with going behind her parents backs, but he has to make the choice and be prepared for the consequences of her being abused if caught(if that really is what would happen..by him staying with her, he is leaving that as an option - leave that weight on his shoulders).
If you call her parents, you might just ruin your relationship with your son. and who knows, maybe they'll break up in a month, or maybe a year, but you never know if it will last. They are just teenagers.
(sorry if this doesnt help, just my side)