OP you’ve given not a lot of information. How long have you been together? That’s quite important. However you are married. That is a long term commitment.
I don’t understand all the posts saying that this boy should be allowed to dictate terms of seeing his Dad, completely ostracizing you in the process. You are his Dads wife. You will be in his life possibly for the rest of his Dads life.
At the moment you’ve pointed to massive feelings of resentment towards you from the boy. Most of them you say are untrue. You are being scapegoated as the cause of his parents not being together.
The boy is young, so not a teen. On one hand that means he deserves a lot of slack, he’s a child, he’s confused and loyal to his mother. None of these things are his fault.
On the other hand, he’s unlikely to be able to overcome even partly his resentments and could well refuse to see his Dad as a teenager, and feel angry to his Dad for a long time. As a teenager he will need as much positive parenting as he can get.
So I think that there has to be some challenges to this boy, for the boys sake as much as anyone else’s. Yet it is crucial this is in a way he can cope with. If he isn’t challenged, he will retain his anger (unhealthy for him), will be confused as to the man he grows into himself, and feel loyal to his mother to the point that he takes on protection of her, by taking out her anger through him.
On the other hand the priority is that he has a very solid relationship with his Dad. Lots of consistent, loving, parental contact. It could also massively help if his Dad did everything he could to make a relationship with his Ex better, so the child didn’t feel so conflicted.
I wonder whether a good counselor, someone neutral that the boy can confide in, would help?
And seeing SM even just once a month, for a meal, where he has to at least be polite, will help the boy to diffuse his misplaced feelings of anger and see that the OP is just a person. Not a monster. This is important as cutting her totally out will result in a life long barrier that cannot be broken down, which will affect his relationship with his Dad. And he needs his Dad.
I’m partly saying this as my DP allowed his daughters to ignore, ostracise and blame me over the years, and it’s resulted in them never visiting our house, and not bothering about our child and half brother, and excluding their step brother. DP never tackled them, or expected them to just acknowledge me. He enabled it. It’s caused lots of problems and most of them now are between DP and his daughters. Their relationship isn’t great. Despite DP doing everything they wanted, he takes them out everywhere, he gets them meals, cinema, sees them daily as he gives them lifts. However, because they refuse to acknowledge that he has a life with a partner, they have lost a genuine relationship. All around. Sad really.