What rights do I have regarding my 15yo son who is allowed to spend nights and have sex with 16yo girlfriend at his moms and also her parents homes?

Unfortunately, I see little that you can do that will be helpful in the way you want help. If you raise these issues formally, your son, his girlfriend, and perhaps his mother and her parents might get into some trouble, but it certainly will not lead to your son wanting to spend time with you. At his age, regardless of the court orders, kids will end up spending their time where they want to spend their time, so you are unforutnately very unlikely to get more time with him unless he wants that. Sorry. I would be very careful here unless you want your son to get into trouble that could follow him for a long time.