Even from his cell, Ozai hears the news of the nation. He doesn't ask questions because it galls him that he has to ask, but he has ears to listen.
He doesn't like the peace, or the new people and ideas coming in freely, rather than subjugated. And, yet, the nation is prosperous. Neither the nobles within nor the barbarians without seize on this new weakness.
The most galling thing is that the boy doesn't even realize this was a subtle form of the revenge of the unfavored child -- to have surpassed his father.