Be nice to his girlfriend. You are under no obligation to like her. But you absolutely are under a social obligation to be courteous and gracious toward her. (At least that's what proper etiquette would advize.) Do not speak disparagingly of her to your son.

I give you credit for having the honesty to recognize that your relationship with the gf is at the crux of the alienation that you feel between your son and you.

Your son may have made a bad choice of a domestic partner. But that's for him to figure out.