Trying to separate a son from his mother is a road to nowhere, even in healthy families, with a parent who's NPD he'll have suffered all manner of guilt and shaming comments from her that will have had a lasting effect on him and keep him emotionally tied to her through guilt-ridden feelings, so you are stepping on dangerous grounds in trying to come between them.
I'm not saying you are wrong, but I think you need to tip-toe lightly. From your post I suggest the measures you've put in place to distance yourself from her are about the best you're going to get.
You've already experienced him trying to please you with promises of detaching from her with the 'really bad' comments, but find that he changes the goal posts when she does do something 'really bad' and minimises it. This is the cycle him and her are in and if you persist on trying to get him to see her the way you do, you run the risk of ending up being the 'bad' guy, or worse, you being labelled the 'crazy' one.
Good luck, you'll need it...