((Granite))

Forget about the parents for a moment. The important thing is your son's girlfriend. It would be worth an effort to reach out to her. She probably thinks that you don't like her. I'd put money on it. If you could make friends with her, everything else would fall into place. She'd feel welcome at your house and your son would come to visit you.

I know this won't be easy for you, but the stakes are high. Worth a try, I think.