The mayor is well respected that even the mother and father told Ultos to respect the guy. If he had just left, he would bring shame to his parents, cause it's like the mayor wasn't accomodating enough for Ultos. And if he reported the mayor, they wouldn't believe him, despite turning his image around, he still has a stain of being a brat and the mayor can just say Ultos doesn't like the mayor and is slandering him.
While not offending the mayor is important because he is powerful and has a lot of influence above and below the table, If the mayor were to say "Your ungrateful brat left the diddy party i hosted for all the little noble kids" he(the mayor) would be in a lot more trouble with high society than ultos would.
The real danger would be the mayor getting offended and having the prostitutes (and maybe the child nobles he has fresh blackmail on) start a rumor that ultos was doing vile things at or hosted the diddy party. This would bring shame to his family, but if Ultos was socializing at the gathering like he should have been, he would have a solid alibi to be able to combat these rumors, since there would be witnesses to him not being at the diddy party and atleast a few people convinces of his changed nature to speak well of his character.
Going this route still has him end up on the Mayors bad side potentially, but none of the "Ultos is a scumbag" allegations have as much weight.
But now since he's actually been at the diddy party for a long (I think) amount of time and he's conversed with another noble (who will most likely substantiate that Ultos was at the diddy party if asked) He's got no alibi and witnesses to his participation in the event. Even without him being caught in the act like in the game this alone is enough to put a significant dent in his family's social standing.
Really hope what every our boy is planning to get out of this makes a nice middle ground where he takes himself off of the mayor's radar and avoids misunderstandings because right now his plan really just seems ill-thought out.