It does not sound like he’s in a cult. It sound like he’s with this girl and her parents.
Have YOU seen a therapist? You should.
I think as hard as this may be, you may need to stop trying to contact him for a little while. Your husband is the point of contact with your son now. It’s clear he’s having issues with you and his viewing your behavior as control over his life.
He’s an adult, and he can make choices himself. He’s trying to maybe show you that, in a very messed up, hard to swallow way. I would really encourage you to reach out to a therapist for yourself. They can help you work through scripts to use, to help you negotiate and communicate better.