Well, I'm going to go against the grain a bit here & say yes, I do believe it is
possible. Not easy or quick or rational, but definitely possible.
But first he has to acknowledge that these relationships are triggers for his anger & seek to manage and/or change it.. Then he can get around to Acceptance- accepting that they are not seeking change & that this is the best their relationship is ever going to be. Then, he'd likely find it necessary to create some tough boundaries for himself & enforce them...... but until he Accepts them as they are & recognizes their relationship as toxic & damaging, it's just you Mama Bear-ing him.
I know how it is Stung - my RAH's FOO is so toxic to him that I used to want to physically pull my hair out after he had interactions with them. Anger wasn't his issue - he'd slip into dark depressions & "miss" this family that only existed in his mind.... the reality of who they are was never what he wanted to believe of them & then every simple conversation set him off into a moody depression that could last hours or days. And he kept going back to that dry well for water again & again & again & again...... it drove ME bat$hit because I could very clearly see the pain it was causing him.
I (like you already have) figured out I could only create boundaries for myself & DD where they were concerned & that he had to put on his Big Boy Pants & do the same for himself. I could not control it for him & I also realized that letting it get to me on the level that it did WAS my fault, it didn't have to be that way. Just my $.02.
When he finally realized that they are never going to change because this is WHO they are (especially his mom) then it got a little easier each time, but it has literally taken YEARS to get to this point. He's had to accept that their relationship is just a bunch of "fluff" at this point, he's stays pretty detached & they talk about shallow, surface stuff. He may still have to address some issues in the future with his mom especially - he has only just started to dig into some of his deep, deep damage where she is concerned but for now he's happy keeping her at arm's length & not allowing her to have contact with DD.
When he first created & enforced his boundaries with his mom she retaliated by going NC with him for 2 years in an effort to punish him.
It was the BEST 2 years EVER, lol!