Since I don't know your son, i can only tell you what happened in my case. ...I didn't care for one of my son's girlfriends and when he asked what I thought, I told him the truth...but also told him it wasn't for me to decide his future with this person, just to be careful.
Sooner or later, if the girlfriend is everything you say, hopefully your son will wake up and realize what kind of person this girlfriend is for himself.and that will be the end of the relationship. It might take a few years (like it did with my son) but as long as there's no marriage, there's a chance he is waking up.