You have to decide if you can overlook your boyfriend not liking this aspect of your mother's personality. Even if you two wind up as a permanent couple, your boyfriend has no obligation to like your mother. His obligation is to be courteous toward her. She has the same obligation toward him.

If your boyfriend brings this up again, you can explain that your mother is as she is and that there's not much you can do about it. Tell him you appreciate any patience he can have when he's around her.

I'ld be kind of slow to tell my parent what they need to work on. Now, if she talks over you, you might try, gently, to say you need to finish your thought. If she thinks this is coming from your boyfriend, she will sorely resent him forever. You don't want that. Good luck.