"Waiver" is a noun. The author is using the verb "waver." Augustus' courage didn't change, vacillate or diminish. It stayed consistent. It didn't waver. He always stayed courageous.

In the context of the sentence you provided from "The Fault in Our Stars" by John Green, the phrase "did not for a moment waiver in his courage" is essentially saying that Augustus Waters never lost or doubted his courage or bravery. The word "waiver" here is likely a typo or a mistake, and the intended word should be "waver." To waver in one's courage means to show uncertainty, hesitation, or doubt about one's bravery or determination. However, in this sentence, the author is stating that Augustus Waters remained courageous and steadfast throughout, without any moments of doubt or hesitation. So, a corrected version of the sentence might be: "According to the conventions of the genre, Augustus Waters kept his sense of humor till the end, did not for a moment waver in his courage, and his spirit soared like an indomitable eagle until the world itself could not contain his joyous soul."