PLAN a guys night.....that might be guys & dads.....family, neighbors, friends, etc. It doesn't have to be huge, even 6-8 would be good.
Have you talked w/ this girl's mother and/or parents? Sometimes you get a much better understanding of what's going on and can deal w/ it better if you have the whole picture. I know I did when I taught school. It also helped me handle the child differently when I knew what that background was.
Lastly, you need to have a SERIOUS chat w/ this kid about what he wants in a mate. He needs to be able to LIST these qualities. He needs to be able to list what he can bring to a relationship. Sometimes, you need to see things in front of you before you get it. This is how I FINALLY decided to end an off & on (several years) relationship. As much as I enjoyed the relationship and we had SSSOOO much fun.......each of us had too many issues to work thru to bring into a relationship. As much as we cared about each other, it probably would have destroyed us. Once I SAW the list of +s and -s, it wasn't even an option.
You also need to set some guidelines and "rules" about his dating. He DOES live in your house so he needs to honor those (honor thy father & mother) until he's in college and on his own. i.e. "nasty attituedes will NOT be tolerated" (by ANYONE in the house, including mom & dad) Have a solution as to how you're going to handle and an alternative as to how they COULD have been handled. Talk about how these things effect everyone else. Sometimes people just don't see that. You need to have a converstaion with him and another one with BOTH of them. I hope it's not too late. If you set these standards and ABIDE by them then there are no ?s or grey areas.
You need to talk about how he values himself. Sounds like he might think this is all he deserves. If none of this helps, I'd FORCE them to go to pre-marital counseling. They will set things straight and many times have them do exercises that will end up dividing them permanently as WELL AS a GREAT learning experience.