Kea-

The whole situation sounds very similar to my grandmother (dad's side) and my mom. Mom and Dad started dating in high school, and Grandma never took to Mom, for some reason. It was pretty bad. She did the whole "accusing the inlaws of stealing him away" thing, too.

There were many uncomfortable family get-togethers over the years, arguments over the phone, and that whole business. Then Grandma finally decided to move to another state without informing my dad or even leaving him a forwarding address (although she did tell some of his siblings.) He took that as a sign and finally decided to break things off with her. My family hasn't heard from her since, and I expect she doesn't miss any of us very much.

Not saying that your BF needs to completely sever all ties with his mom, but just be sure that if his mom gives him some kind of ultimatum that you come out on the winning side. Also, while you can't pick the family you were born with, you can usually remedy that as an adult.