We (and the MC) knew that he was a sadist (S) ( = want to inflict pain to the masochist), but not that he was a dom (D) ( = want to dominate, the life of his sub).
They were into s/m plays (sadist/masochist), but definitely not in a D/S (dom/sub) relationship, wich has much consequences.
That wasn't the deal and nothing until then show that the MC being dominated. To this point he just like pain and restriction (and only during plays).
A to answer your question, he wasn't calling him mr. cha because it wasn't a s/m play but him being a (very very very bad) dom and being angry at his (never proclamed) sub for not obeying him and accepting being controlled in his own life (wich he never agreed).
Ok. You explained it perfectly. Usually i read bdsm mangas where they mix the 2 concept but as u said they are different. It just came to me
Anyway the dom sadist ecc is shit. And i would have loved it if "mr cha" would have used the safe word or went away. Cause he went over board without consent
wait, why him being a dom is a shock??? wasn't it obvious???' i m confused. also why wasnt he calling him mr. cha when he was punishing him. that's against their rules and their play. i know the uke is a total pervert masochist ecc. but stilll, i hjated these scenes. i didnt see this as play at all