I'm so sorry this happened to you. It's obvious he's thinking with the wrong head, as probably most young guys do. Good news is time is on your side! I might try to "embrace" the girlfriend, that's what I did with the boyfriends my girls had, like them or not, and and you know what, they both ended up marrying wonderful men. Letting them make their own mistakes gave them the confidence to make a good choice, when the time came.
All you really want is to be in his life with no drama, so work towards that end, with the help of your BF. I would go to the races, be super friendly to everyone and let her show her true colors, as she no doubt, will. Good luck!