his mother is dying. Mia isn't just dangling the new medication over his head, she's funding his mothers' treatment entirely. we only see her refuse the new treatment because she realizes something is awry, but he is still dependent on Mia financially and this is her bargaining chip
this is not ego; he is not thinking of himself in this. it's inconsiderate at worst but it is clearly just a child's desperation for their parent to live, as well as a poor mans plight against his circumstances
and how do you even expect that conversation to go? he clearly feels backed into a corner and is being taken advantage of both sexually and financially. she pissed in his mouth to scare Jung-Woo away bro, she calls him Myung-Ha's 'little boyfriend' and gives Myung-Ha an ultimatum, demanding they break up or else she'll stop paying for his mothers chemo. he's a possession to her. no shit he didn't try to 'talk things through'
and thought "omg i'm so dumb at least my boyfriend is not selling his soul to an evil woman gotta support him and be grateful"