Well, if it makes you feel any better ... I did have some similar problems when Mike and I were first married. His ex left his mother believing that I was the reason they split up (even though she had moved 2000+ miles away a month before we met).
So initially, she used to talk about the ex to Mike like I wasn't even there. She felt so sorry for her ... she gave her extra money because she just couldn't make it on her own ... she felt so bad that she was crying and missing Mike ... yadda, yadda, yadda. Mike had never told his mom what happened because he didn't want her to dislike the ex since she lived fairly close and visited his daughter frequently.
The real kicker was right after Jessi was born, we flew out to visit his mom, and the ex brought Mike's daughter down to visit. She announced that she wanted us to ALL spend a day together (which just happened to be my birthday!) and when I refused, my MIL couldn't quite understand why.
So I finally put my very hormonal post-partum foot down and told DH that he could tell his mother the truth, or I would. He finally told her the story ... the whole story ... and that completely changed things. Ever since then, my MIL and I get along great and she rarely mentions the ex in front of me, unless it happens to be a conversation about my SD.
I guess what I'm getting at here is that if you don't demand he stand up for you, he probably won't. Most men are "mama's boys" and have a problem standing up against them. Perhaps you just need to remind him who he's going to have to spend the rest of his life with. LOL