Well he has been having sex with his girlfriend the baptized pioneer elders daughter
Probably doesn't matter, but I'm a little confused. Is the girl a baptised pioneer who has an elder for a father? Or is she not a pioneer? Her father's both a pioneer and an elder? Who's the pioneer? And also wondering, are you an ex-JW? Or were you never one?
If she's the pioneer, wow! I always thought of them as being the most brainwashed robots they had. Real holier-than-thou types. For her to date an non-baptised guy is surprising, and even more so to let it get as far as having sex.
Back in the congregation I was in until my mid-teens, we had a guy who did *something* to an elder's daughter - probably a lot of horizontal grinding with the clothes on (according to the "needs" talk afterwards). He got DFed, she got reproof. He turned out having a pretty moral life, she stayed a JW, married some dork (probably under threat) and ended up cheating on him. She even approached this DFed guy and tried to have sex with him, but he (though single) turned her down because she was married. Leaves me wondering who was the aggressor in the original incident.
While her father may not be happy with her, he may use his elder status to protect her from being DFed. Your son, on the other hand, not being baptised can't be DFed. I forget what they can do to him. I think there's a form of "shunning" that's not the same type of shunning as being DFed. As I remember it, people might decide he's "bad association" and may say hi to him, but stay away socially. As for her (assuming she is the pioneer one), she's going to fall a lot harder. Even if she gets off light, she's going to have to stop pioneering. That's a status symbol for JWs and having it taken away hurts.
Like others, I'm glad they didn't get married just to find out what sex is like. Straight out of high school is way too young for that. Hopefully your son's trouble will cause a seperation between him and the borg and he can free himself. Hopefully, she can too.