I think Seungho's mad because he thought Nakyum had some affection for him but then Nakyum said that thing about why Seungho is keeping him around. And then Nakyum started behaving like that (thanks to the server guy) so that kinda reaffirmed the thoughts that have been going around Seungho's mind...
I don't think Nakyum acting obedient and submissive to Seungo all of a sudden is thanks to the server guy.
I think it's because of what the learned-sir said to Nakyum about how he was just a filthy whore.
Obviously considering Nakyum has loved and looked up to the learned-fuckhead for such a long time, he took those words to heart. Hence why he's acting like this now.
That server guy is just rubbing salt to Nakyum's wounds by his mistreatments and name calling.
I thought this would just be all smutty and cutesy but I didn't expect to get a Saezuru Tori Wa Habakatanai-sh story lol