I'm not sure how helpful I can be but I'll give you my limited perspective on what you have described below.
It seems like you've tried a number of ways to work on the issues that you have within the relationship and they have not resulted in any improvement. Have you tried getting psychological support for yourself? I'm not implying that I think you have mental health problems but it seems like you're stuck in a loop and are finding it tough to know how to break out of it. Psychological support could help you to work out what the right way forward is for you, your son and your partner.
Unfortunately when someone doesn't want help or doesn't think they need help there is honestly nothing you can do or say to change their mind, all you can do is give them information and tell them how things are affecting you. With that in mind it's important for you to try to be realistic in all of this, I know how hard that is when emotions are involved.
I'll tell you one thing that I know from personal experience: staying together for the sake of a child can actually do more damage than good to the child. I am a child of a narcissist and my other parent stayed with them for the same reasons you have said. I have a terrible relationship with both parents. I have no relationship with the narcissist and my relationship with the other is fraught with resentment. I resent that the other parent exposed me and my sibling to a selfish and deeply self centred person and basically validated that behaviour by staying with them (they are still together now by the way). It was a terrible upbringing. I know my relationship with the other parent would be healthier and more loving if they had showed themselves and me and my sibling that mutual respect and compassion are vital components of a loving relationship. By staying with them the other parent showed us an incredibly unhealthy model of love. It has seriously damaged both me and my sibling.
I'm telling you my experience above to give you a different perspective of what can happen when people with children stay in unhealthy relationships. Obviously your situation may be vastly different but I ask that you refrain from making the assumption that staying together is always better for the children.
I think with good psychological support you may be able to work out the way forward that is right for you and your son. You could ask your gp to refer you or refer yourself. Or if that feels like too big a step, do you have any friends and family you trust and can confide in?