I read your other thread, and I feel for you.
Why would he go NC on his mum? There is no real benefit to him in doing so. If he stands up to her, goes to therapy to deal with his issues, or goes NC with her, two things will happen:
- He will have to admit that he has issues, and address his "problems" (that he does not see that he has)
- He can no longer play the "poor abused man with a terrible upbringing card"
The problem I see is that he truly is his mothers son. The apple does not fall ... etc...
The current set up suits him perfectly. He has you on eggshells in his home, and in your home together. He gets to subject you to her, so you are doubly abused. He also gets the sympathy card from you, in that you pander to him, try to make things better for you, etc. 3-4 times per year he gets to treat you like shit, and I think you will find that this ties perfectly in with "the cycle of abuse", good periods following bad periods. And as you have discovered, he is acting differently with his friends, so he can control his behaviour, ie he chooses his behavior towards you, and he sees nothing wrong with it.
What do you have without his mother? An abusive and a controlling man. With his mother? The same, but her in the equation too.
In your shoes I would run a mile. Can you take some time off work to flat hunt nearer your workplace, but in easy commuting distance to your parents?
It is good that you are not married, and that you have no children together.
Can you imagine him with his moods caring for your babies, as a part time worker/stay at home dad? You and your future kids would be totally trapped. It is really good that you are beginning to see the light now.
Is your work in/near London? (Im in the SW area)