He's right that he can't get in troubl as long as the sex was consensual--you can legally consent to sex when you turn 17, and even if you were between 14 and 16, he'd be okay as long as he wasn't more than 3 years older than you. What you've described doesn't sound like anything that would be classified as a terroristic threat (saying he's going to kill or hurt you physically). Beyond that, it will depend on what the specifics are whether he could be in any legal trouble for it, but I wouldn't count on it.

BUT... whatever's going on here, just deal with whatever he tells your dad (if he actually does anything at all--he may be bluffing) and get him out of your life for good. That kind of behavior is abusive and very often escalates to actual physical abuse. I don't know what it is he's asking you to do that you don't want to, but don't give in and make him think he has that kind of power over you. That's essentially blackmail, and once it starts, it rarely stops on its own or has any kind of good ending. Good luck.

It is not my habit to respond to a posting when I agree with an attorney who has already answered the question, but here I am going to make an exception. You should read Ms. Foley's second paragraph several times. She is exactly correct. Whatever he might tell your dad, it is unlikely to be worse than what you are facing if you give into his demands. Realize that he is abusive and get him out of your life forever. It would probably be better to tell your dad whatever it is yourself, if he is really going to do it, but that is your decision.