It doesn't matter that what she thinks doesn't matter to you; it does to him. I would sit down with him and your mother and have a frank conversation. That way he could learn directly from her what she actually does think about him and/or maybe realize her opinion isn't as important to him as he first thought. It sounds like she can't have much of a "correct" opinion about him and he should be able to see that; likewise, if you are glossing over some of his imperfections (everyone has them) that might/should matter, you can see that too and you and he can work on those. Perhaps he doesn't like his mother (or his mother doesn't like him) and just assumes all mothers don't like him, for example.

I married my husband, who has no sisters, because of how he treated his mother, ex-wife, my mother, etc.; I could tell how he would treat me by how he thought about and treated the other women in his life.

It's another aspect of him, what he thinks about your mother (and she of him); maybe get to know that aspect better and what it says about him.