Boy is this story familiar!
My wife and I almost broke up over my parents' inability to accept her, and my inability to show her that she meant more to me than my parents.
He MUST Choose
That's really the bottom line in my opinion: You have got to mean more to him than his parents. He must choose. I think that ultimately, it's how we really grow up. We have to choose our spouses/significant others over our mothers (for men) and fathers (for women). It's not always easy, particularly if your boyfriend is really tied to his mom's feelings.
What did we do? We went to a lot of therapy. Here were my phases: resenting my wife for not getting along with my mother, resenting my mother for not getting along with my wife, withdrawal from the whole situation (alienating both my mother and wife), trying to play mediator between them (big mistake!), finally realizing that my wife and my children were the most important people in my life.
I Didn't Pressure Her to be With My Family
The best thing I could do for my wife was to not put pressure on her to do anything with my family, but to let her know that I would like her to participate.
She has slowly started to come to more things, and to accept my mother for all of her faults. I also made it clear to my parents where my loyalties were: with my wife.
So, I guess the ball is in his court. If he can't be up front with his family, maybe he isn't ready for the full commitment to you. Until he realizes that he must choose you first and foremost, you will always feel slighted and not included by him.
If he's not comfortable talking about these big issues with you (and I understand that!) I think you two should see a therapist. He needs to do a lot internal sorting out...