How to teach my son that he is in the middle of a storm he can't control or influence?
That was hard to read. Your poor kid. Echoing what was said above and what I can see you already know, that this will do lasting damage to him. But an interpretive lens from a therapist and from you can help him externalize the source of his pain and not take this on board any more than is unavoidable when your mom talks to you like that.
Most important and most in your control is that you talk with him privately and affirm that that is not OK. That he is not the cause of any problems with his mom or with your marriage. That he should not be tossed out of the house for not doing homework--setting aside the homework issue, home needs to be a safe place for him. Directly criticizing his mom may prompt him to defend her and be seen as you somehow getting something out of attacking her, but there is a way to acknowledge what he likely already perceives--that she sometimes feels bad inside and processes that unconsciously in ways that scapegoat him or others close to her, and that that is not OK, and that you are trying to figure out how best to deal with it. Then give him space to talk and explain how he feels about it. Then validate his experience.
The point you made above, that her way of seeing things is distorted and he can't control or take responsibility for that--also is an important message.
Kids going through such a thing without a trusted adult confirming that there is something wrong with it are suffering double abandonment, from the parent directly inflicting the torment and from the parent standing by. Your words quoted above are eloquent and if you share that perspective with him, he'll have a reliable lens to use to process all of this and will have some reason to think he is not crazy and you are working to make things better--even if you don't fully know how yet.
For therapy for you or him: some therapists will work via Skype or phone under these circumstances. Less than ideal but better than nothing. Seems really important for your son to have someone who is "just his" to talk with, but also important for the therapist to have background from you on the dynamic.
My heart goes out to you. My kid's dad does similar harm. Fortunately we split when she was little and he has very little time with her, but he still manages to make it all about him, he uses her to make himself feel better, etc. Worst is hearing from her how much she wants a "real dad" and knowing how much it would help her if he could actually listen to her, hear her, show up for her emotionally. He can't, though, and me confirming that without unduly attacking her dad (and affirming what good there is to affirm) at least appears to help her navigate her landscape and trust her own knowledge and reactions.
I understand the worry about leaving your kid 50% of the time in an unmitigated exposure to someone like that. I will say that having some zone of safety is super important. If you can genuinely provide that for your son while still in the marriage that may be best, but if you can't, the importance of having a piece of life that is not crazy and hurtful can't be overestimated.
Sorry; I know how tough it is.
