My boyfriend (19M) has changed his perspective of me (19F). How can I handle his mother's influence on our relationship?

My boyfriend (19M) and I (19F) have been together for four and a half years. Lately, things have become very unstable, and I believe his mother is a major influence on this change.

His mother is actually one of my teachers. While she is friendly in conversation, we aren't close; we are basically just acquaintances. She is very religious and projects a "good listener" image, so many people—including myself—often go to her to vent about their problems.

However, I’ve discovered she talks about me negatively to others. She says things like I "have a negative attitude," that I "rely too much" on her son, and that I can be "resentful and possessive."

The real issue is that my boyfriend has started to see me through her eyes. He now treats me as if I am incapable and too demanding of his time. He has stopped showing affection in public, like holding my hand, and he no longer prioritizes our time together. Every day after school, he immediately goes to her. We used to have scheduled dates, but now he only meets me when he knows his mother is busy with her partner or family.

She even admitted she didn't think we would last this long, dismissing us as a "superficial teenage relationship." Everything has changed drastically compared to how we were just a year ago.

*My Question:*

I want to save our relationship, but he defends his mother fiercely and sees her as completely innocent. How can I talk to him about his change in attitude and his mother's influence without making him defensive? What can I do to get our relationship back to how it was?

*TL;DR:* My boyfriend of 4.5 years has started seeing me through his mother's negative perspective, calling me weak and dependent. He now prioritizes her time over mine. How can I address this influence without him getting angry or defensive?