I’m sorry about what his mom said to you. Some people have very ignorant views about what matters in a relationship. Mom is very caught up in ethnicity and status, as if either were an indication of how well you treat her son, which is what should matter. She’s entitled to think what she wants. But she’s all-the-way wrong for saying it to you, especially at an event.
Anyone would be upset after being told to her face that she isn’t good enough. You have every right to be, and if your date said this to you, I would applaud you for leaving and encourage you to stay gone from that relationship. That would be an occasion where storming out is entirely justifiable and acceptable.
But what you’ve described above is not that situation. His mother was out of line, and while she raised him and shares his last name, she is not him; nor is she his representative. You didn’t have an issue with your man; you had an issue with his mother. You took it out on him even though he isn’t the problem.
He obviously feels that you’re good enough, since this “feud” has been ongoing and he’s still proudly bringing you around his family. He isn’t swayed by what they think—or he wasn’t. He may be now.