I am so sorry to hear that you guys are going through this. Sometimes it's impossible to reach people; once they have their minds made up. My brother went through the same situation, his wifes mother decided she didn't like him and kicked her daughter out, etc. Needless to say, she wasn't invited to the wedding. It took a few years for my sister in law to talk to her mother again, but now talks to her on and off. Her mother still continues to pull stupid stuff.
Is his mom one who likes to control things? "It's my way or the highway" type of deal? It's like she's close w/him when he does things her way, and if he doesn't, she gets mad?
But this is the two of you starting a life together. His parents have 2 choices; they can accept you and support their son or they can be stubborn and lose their relationship with their son and lose out on getting to know you.
I hope everything works out for you.