Look I'm a sm but generally thought this was a reverse.
Your partner created a child and that child is disabled. He should be doing 50% of the care and is doing more yes should claim the benefits and house him accordingly.
Two disabled children in the house is a lot. One needing a lot of assistance and his mum not being very hands on with him is really heartbreaking. I'm sure i hope you didn't mean it to come across the way you have.
But honestly as much as I think the evil nutter mum trope is wheeled out to much, I also cannot believe you said "feel like he's choosing his past life over me and having a new family" it feels like something read from the wicked sm handbook.
Holidays should be split. Your partner is the dad so he should be splitting it with mum fairly.
I can imagine this is a lot to deal with, but I really think you should look at parallel parenting strategies- basically only focusing on what goes down in your house and ignoring mum (bar when dad is sorting out childcare)
This feels like a DP problem, because I imagine your the one doing most of there caring for dad while he steps back.
So mum is the red herring here imo.