
in my opinion, i think he just fell in love with the master and the feelings he gets trembling and stuff is because of that. it's just that he can't admit to himself that he's in love with master, he blames the thing he drinks.
but it'a just my opinion ( ̄∇ ̄")
maybe i missed something in the storyline idk this is the thing that came to my mind first

They are not aphrodisiacs. It is medicine the servant/Seungho ordered for body strength. Nakyum's hidden desires are fulfilled by Seungho, so he's attracted to it. I don't think he is in love with Seungho yet but he might eventually think he is just because he is being spoiled through sex. Sex is freedom for Nakyum, thus Seungho is freedom.
Wtf, giving Nak yum afrodisiacs is so fucked up.
I really was rooting for that servant wtf is he doing