Hmmm, sounds tricky for sure. I think it could have something to do with the fact that he associates her with you leaving, but probably not completely. My advice would be to take charge and make it happen for you all to be together more. You may have to do dinners together, play dates, etc. If you live so close it is really a shame she can't be more of a help. I think that you're going to have to visit her house even if it isn't childproofed. I am under the belief that your child needs to learn (within reason) what to touch and not touch and should be able to go most places regardless of childproofed or not. I have two little kids and I know that is hard, but I think it's mostly possible. YOu can give her a heads up and say hey, I think we'll stop by on Wednesday, just letting you know in case you want to stash your valuables..haha! If she's uncomfortable with you all coming over, that's a different story...I would just have her come to you or meet in neutral locations.
Also, and I know this is going to sound really horrible, but maybe she can start some sort of tradition with him that will make her more appealing. Maybe every time she comes over they can play a special game or eat a special treat or watch something on TV, etc. Then maybe he can associate getting something he wants with her visiting. Like I said, horrible, but I might not be above it! I know I would want my MIL to feel loved and welcome in my home, especially by the grandkids!
I do think you need to sort of mention something to your husband, like...why do you think he acts like that? What should we do? Should we start hanging out with your mom more? Maybe make him believe that some of the ideas are his and go from there!