How is he faking it?

It is still possible to become tipsy with just a cup though knowing that Jaekyung doesn't drink at all. And if the dose of alcohol is harsh, then yes he can be drunk seeing as he gulped the whole cup and started coughing afterwards. I don't think he's faking it bc why would he say, “fuck, I thought it was water” at the end of the chapter? For what purpose would he lie and pretend? That seems off. He can easily have sex with Dan if he wants to cause that's their contract. I certainly do not think he's faking it y'all.