"Nothing." Bumi's answer is crisp, commanding, and almost immediate.
Possibly more so than it strictly needs to be at this moment, which is why he makes sure he's at eye level with Aang before he says something else.

"There is nothing wrong with you. Your bending is a different story, but you are not your bending, and there is nothing wrong with you."

His head tilts slightly, silently asking if Aang understands. If the boy is capable of seeing the distinction.