MouMou, as with all MIL related issues, I think the real problem here lies with your DP - as far as your relationship and your life and family are concerned, I mean. Clearly the problem in general originates with her!
The problem is that he is in massive, massive denial - it is nothing to do with him "being the better person" or truly forgiving them, and everything to do with him literally not being able to bear to face reality. He cannot acknowledge that his mother is a vile bitch, and has spent all his life scapegoating him. It is, after all, pretty hard (read, unbelievably hard) to acknowledge that your mother is a vile bitch who scapegoats you.
So he does what abused children always do: puts the blame on himself. "if MIL found it hard to love him as a child it was probably his fault" - you are dead right to find that totally shocking. It is. But it's also what normally happens after prolonged EA. It's what he did as a child to cope with it all, to survive it.
Some people grow up and start to challenge this stuff, and some don't. Some remain in that state of denial, of being emotionally a child for the whole of their lives. Some people spend their whole lives exonerating their abusive families, and taking the blame on themselves. Which is incredibly sad.
And it's also very destructive to healthy relationships. As you have said, he is blaming you for the NC situation - that's because he cannot face what blaming the real culprit would entail. That is not great news for your marriage, I'm afraid, IMO, and as you also say it's leading you to punish him. Unfortunately he is "unable to truly protect himself from them or their lack of love and abuse" as you say, and I suspect it goes so deep in him that he is also unable to protect you or your DS from them either.
By rights you and your DS should be his number one priority but I believe that when there is a background like this, his number one (unconscious) priority is staying part of his family unit, being one of them. He would probably deny that if asked, would probably not even realise it himself, but my guess is that that's the case.
All of which leaves you... I don't know where, sorry, I'm not great at the advice part! All I can say is that you are right in all your instincts, and he is the one trapped utterly in FOG, and you need to be aware of that, and handle your relationship accordingly. You are seeing things as they are; he, at the moment at any rate, is incapable of doing so and is repressing all his own anger and hurt. But he thinks he's being calm and reasonable and that you're the "difficult" one, and that could make communication very difficult.
I guess it depends on how strong your relationship is otherwise, how you proceed from here. And also maybe it would help for you to have some support around this and see if you can separate any of your own baggage from your own toxic mother from the stuff around your MIL, as I would guess it might be triggering you. It is horrible to be this angry and upset about stuff and to carry it with you while the real villains go scot free, but life sucks like that.
Anyway, hello to all and apologies for jumping on the thread with a diatribe like that and no introduction, hope it was at least helpful in some way. I've had quite a lot of experience of dealing with vile parents and scapegoating and so on myself, as you can maybe guess, so I thought I'd just throw my two penn'orth in in case it's any use.