If a much wealthier friend insists on treating you to dinner every single time you go out, is there a point where you

If a much wealthier friend insists on treating you to dinner every single time you go out, is there a point where you must insist on paying—or is trying to pay actually offensive to them?

My best friend is a successful surgeon, and I'm a public school teacher. We hang out weekly, and he always just grabs the check and says, "Don't even think about it." I've tried to slip him money or grab the tip, and he gives me this look like I've insulted his ability to be generous.

Is there a subtle, non-offensive way to say, 'I value our friendship, not your money,' or should I just shut up and enjoy the expensive sushi, knowing I'm doing the right thing by letting him treat? I feel like I'm breaking some unwritten rule of friendship etiquette.