oppa?

He says something like "your oppa will make you feel good" probably its a way to treat him just like in yaoi manga they use a feminine way to address themselves, thats how it looks from my perspective

Ah yea but I noticed the mc says that to everyone even in the previous chapters

I saw it as a joke sorta thing, and/or something he’s been forced to get used to saying since he had to sell himself for a living, I think bc men like being called oppa

Oppa is the way of girls referring to ppl they deem as big brother either related or not. And in this case uke has feminine features so that's why they use oppa for stimulation and enjoyment.