Boyfriend’s mom straining relationship

Background:

1- My boyfriend and I both still live at home while we’re going through school and training so we don’t get to spend a whole lot of time alone together. (Med school for the win)

2- His mom has medical problems with her back and her weight.

Every time we’re out together he gets a call from his mother saying he NEEDS to come home. Either because he hasn’t done chores (despite being 21) or that she needs him to help her with something she is fully able to do. Now I understand she’s disabled and needs assistance with some things, but I also know she is fully capable of doing things by herself and just chooses to be lazy instead. (I have disabled family members as well so I understand there’s limitations but she is just blatantly being lazy.) And every time he gets upset but goes back because he doesn’t want to get into a fight again. She’s constantly yelling and belittling him about everything. She’s driven off one of her kids already who refuses to come home for family events and I can see why.

It seems to me she’s trying to break us up. I hadn’t seen him for almost two weeks and he was coming over to spend the night while I was dogsitting for a friend after he got off work at night, and the next morning she and her husband called saying he needed to come home to let the dogs outside because she was in too much pain to do so. I don’t want to be a jerk but it’s gotten to the point I just don’t believe her anymore.

If he ignores her calls, she’ll start harassing me. Calling and texting. And I’ve heard her in arguments saying how I need to understand he has obligations to family and that I don’t come first. Or that I’m just some girl and I need to back off. (But of course never to my face.)

This is putting a huge strain on both him and me. I love him and we’ve already decided on getting married after graduation. But I also don’t want to be that woman who constantly hates on her MIL. Even my mother, who met her once can’t stand her.

It’s his mom and I know she’s not going anywhere but I’m getting to the point where I can’t stand her. I don’t know how to bring this up that I feel she’s ruining our relationship or trying to break us up but she’s driving me insane!