Your last statement makes it seem to me as if he still respects her to a degree. That is good start. Chameli, you are doing such a wonderful thing here...reuniting a mother with her child...giving your husband his mother back. He is a lucky guy, hope he realizes it.
I think there might be some room in his heart for her but either he doesnt know it yet or chooses not to acknowledge it. I would think if you even started to put little ideas in each of their heads, it would begin to happen. Like casually mention things..."talking to your mom is so exhausting...all she does is talk about you"...this or similar comments will pique his curiosity to a level and after a while he might start to take interest. Since these bad feelings are years and years old...I would say dont expect anything right away.
Another suggestion: dont address the issue with him head-on. Keep working at this and see how long it takes for him to bring it up to you. It has to come from him because its his mother. If this issue gets raised by you, it could bring a whole host of problems that are unnecessary.
Good luck!
Thanks for your good advices:)