If Runyu was anyone lesser, he would've ordered him/ asked him to make the tea. As it was, he didn't want to draw attention to the fact he was self-medicating more than simply thirsty. Since, obviously, he's doing such an excellent job hiding his weakness so far.

As the tea heats- in a traditional way, not using his spiritual force- he turns back to the dragon. "What was your father? Was he a dragon?"