How do I explain to my boyfriend that I need to financially support my single mom?

Hey everyone, I'm in a bit of a tricky situation with my boyfriend of almost a year. We're talking about getting engaged, but we're having a disagreement about financial responsibilities to our parents. Some background: I'm 27 and work in finance. I grew up in a middle-class family with a single mom, and I help support her and my younger brother financially. My boyfriend is 28 and comes from a more well-off family. He works in his family business and makes significantly more than I do. The issue is that I want to set aside some money for my mom's financial security. My boyfriend doesn't agree with this. He says that as a son, it's his duty to support his mom however he wants, but that as a daughter, I should limit what I do for my mom after marriage. He thinks it should be my brother's responsibility. I'm really struggling with this traditional mindset. I feel like women are often expected to sacrifice for tradition, and it's not fair. I don't see why I should have to limit how I support my mom if I have the means to help her. Am I being unreasonable here? How can I help my boyfriend understand my perspective? Or am I missing something about his point of view? I'd really appreciate any advice on how to navigate this. Thanks!