He needs to develop strong, and I mean STRONG, boundaries with his mother, and that could take a long time and many many therapy sessions. He became defensive and defended his mother when you pointed out that her behavior hurt you. He needs to really absorb the fact that his mother is way too overly possessive of him and dependent on him. Not only that, but her behaviors were intrusive in your relationship! She set out to destroy it so that she could keep him all to herself. That's obvious. You cannot control or influence HER behavior, and the ball is in your ex's court to develop and hold strict boundaries with her. But he won't do this if he cannot acknowledge the problem and face it.. and resolve it.

Honestly, I would move on if it were me. This seems like to big of a hurdle and mountain to climb. It's his mom, and he may not even be able to acknowledge that her behavior is a problem.

I broke up with a guy many years ago for the same exact reason. He was Greek and married a Greek woman. We never were able to see eye to eye about his mother being a problem in our relationship. Same thing.. she was overly dependent on him and he allowed it. So we broke up.