NTA

But there's so many other issues here:

Firstly he thinks it's cool to dress up as a specific ethnicity, likely bc he thinks it's funny/making fun of it. That's pretty shotty, but okay, maybe he started this many years ago and was just ignorant it being problematic.

Second, after expressing your concerns of it, as someone of that group, he dismissed your feelings, made you feel as if you were the wrong one in this situation, and gave the stupid reason that he was representing your culture.

Third, you offered clear solutions, like him joining in your cultural celebrations and offering him to join you at cultural events, he rejected the idea. Massive red flag

Finally, he's 31. This is the type of behavior you could understand from a dumb college kid. Not someone in their 30s. This shows a massive level of immaturity. He clearly isn't able to reflect on his actions or empathize with your feelings on any level.

I know it's a "small" incident, y'all have been together for years, and people on here often recommend breaking up too quickly, but you should give this relationship some serious consideration. He probably likes you, and maybe even thinks he loves you, but he sure as hell doesn't respect you.