I'd just be blunt about it and when he mentions it being "off" I'd just retort "yeah, well it seems a little 'off' to me that you're incredibly obsessed with something like that. Why is that?"

He's just an abusive asshole trying to get your goat. I wouldnt mention that it hurts your feelings, because that's what he wants. I'd just make it a point that he is boring and annoying to keep wanting to having the same confrontation over and over again for 16 years.

Tbh it probably REALLY hurt your feelings when he shamed you for it as a kid (which I'm sure he did) and that's why he keeps going back to it. He wants to hurt you in that same way when you were a vulnerable, embarrassed kid. I think that's sick and it speaks volumes on the kind of abusive cunt he is.

Sorry that he sucks. I hope you have a better source of support in your life and that he can get the hell over himself.