God this could have been written about me and my MIL.
If she is as all all consuming as mine (demanding/ relentless/ self obsessed and generally very unpleasant), he may be at breaking point and cannot cope.
I started off similarly couldn't do enough, put my all into caring for her. She slowly ground me down to the point I do it for my husband only.
Having reached my own breaking point I hate being near her now. I feel bad but I've been through too much, so I take on the mental load and try to interact with her as little as possible.
The difficulty you have is that at the end of the day he is your Mum so you probably have nice memories growing up etc. Whilst he loves you he doesn't have that connection with her or have those lifelong memories meaning his tolerance threshold is likely lower than yours.
Caring for my MIL and dealing with her ridiculous behaviour nearly destroyed my relationship at one point. Much of my own frustration was centred on how she treats my husband which ironically manifested in me having awful arguments with him which sprang from sheer frustration and then made me feel even worse.
In the end I had to step back for my own sanity whilst supporting my husband in my own way.
All I can say is you need to talk to each other if you can. Have a really frank conversation whilst being prepared to hear things you don't like - I've had to be very diplomatic whilst setting boundaries for myself. It hasn't been easy and we have had awful arguments (prior to we barely argued in 13 years) but touch wood, we're in a much better place now.
Good luck