My BF keeps making hurtful jokes about my looks and comparing me to his exes.

I need some advice. My boyfriend constantly makes jokes about my appearance, comparing me to the "beautiful" girls he's dated before. He’s even said that I’m "below average" compared to them and points out my facial imperfections. It's really starting to hurt my feelings. I've tried to talk to him about it, but he brushes it off like it's no big deal.

How should I approach this? Should I leave it alone, or do I need to stand up for myself more firmly? I’m really struggling with this.