Am so dumb can someone tell me whats the meaning of him not drinking the tea??

I’m pretty sure it’s just a respect thing that most “lower class” people wouldn’t know ! The whole “don’t drink before the elder” type of thing I believe. But I could be wrong since it’s been ages since I read that chapter lol

As per the novel, the idea is that someone from a lower class background would never have the nerve to reject tea offered by their higher ups, especially when pressed. In a strictly hierarchical society, a random citizen with no backing rejecting the Duke/ Commander is unheard of. Thus, a boy from the mountains doing this says something about his character, and can also be seen as suspicious/ noteworthy etc.