Infool, I’m so sorry, and I am praying for you now.
I have a nephew who went astray years ago and has yet to come back. From what I’ve witnessed, I advise you straight out: don’t tolerate his behavior, don’t coddle it. That’s what happened with my nephew, and it was such a mistake. Nip it in the bud by showing him you will not accept such treatment.
There have to be painful consequences for terrible decisions, and, while you always and continually pray for him, you have to have zero tolerance for this behavior.
I will pray for you and your family. I'd be willing to bet that at some point the bloom will come off the rose, his eyes will be opened, and he will return to you, more mature and much wiser. At some point, things are going to get tough and the girlfriend will not be worth the unpleasant circumstances and his family will look beautiful to him. He is still very young.
This is a terrible experience for you and I will pray for you.