He already does.
When he was starting to really listen to what the maid told him about the master caring about him, he started to look at Seungho differently and that's why he asked him why he was being so kind when he gave Na-Kyum his food.
But Seungho brought him back down when he said it was because he was too skinny and it hurt/displeased him to have sex with him. Seungho was trying to anger Na-Kyum out of his accepting behavior in an attempt to find out if Inhun set NK up to sleep with him and be more docile. He was also very jealous.
So Na-Kyum ended up, in his mind, confirming that he was just a whore to Seungho.
yiikes
well then I guess this isn't much of a theory as it is law
Inhun influences Na-Kyum deeply. His parting words really marked with him because up to that point, NK was putting up with Seungho's interest to protect Inhun and that A-hole used something he has no control of but which thanks to Inhun and the time, really shames him - his abandonment, low birth and who raised him.
He thinks he is not good enough for Inhun and Inhun basically confirmed how much he looks down upon him.
What if Nakyum starts thinking his purpose in life is to be Seugho's whore? I mean he gets told he's like a prostitute. To which he decided to have sex with Seungho. Then now the thing where he apologizes for being too skinny, and now he gets called a whore. Maybe I've been reading to much weird stuff lately lol.