I really hear you too. These are supposed to be the support systems that are unconditionally there, and they aren't.
It is hard as part of me knows they are a product of what has come before, and their avoidance and lack of ability to cope is the legacy of their upbringing, and having a society where people are not encouraged to communicate on this level.
However we didn't have the coping skills either, but we searched, learnt, got hold of things to help us and our partners. As parents why are they not doing the same.
his mum seems like she's trying to avoid 'blame' - rather than taking on the responsibility of what she can do NOW to help empower and care for her son. Underneath though, you know there will be a torrent of emotions they will be experiencing that they just aren't able to express or work through, yet, maybe never. It is wonderful you are there for him, and you and he together face this with his parents. This was tough for you and you did really well I think.
My partners parents have essentially stopped talking to him as they don't understand why he is so different now. He isn't doing what is 'expected' within his old family dynamic. His mum and he had a hugely co dependent relationship, and now he's wanting to sort his own life out and not need her to 'fix' everything, she's turned on him. I could really say a lot to his mum, but I can't take his voice away, and he will say it if and when he needs to. Not my battle to fight, much as I want to sometimes!. In the mean time I just say it/shout it to an empty chair sometimes when frustration threatens to blow a fuse!