Actually, Ash, he never shown a single care in the world for having his dad sleep around with every women in the empire, not even when his father told it himself. Right now you're just assuming he didn't like hearing about that, so let's keep it factual up in here, shall we?
Speaking about facts, the first time our MC shows a face of discomfort was when his father suggested that his mother was sleeping with his uncle. The MC actually remained very much composed during the segment in which his father said he was not really his parent. When the MC asks his father to appologize, he emphacizes that he doesn't want his father to insult his mother ever again. Once the MC was preparing to sleep, he kept thinking about his uncle (which apparently he already didn't like), not about him not being his father's son.
Don't get me wrong, it would actually make a lot of sense for him to be traumatized IF he had been traumatized for finding out he is not biologically his father's son, but the author didn't emphasize that at all. Instead, he made the MC seem traumatized for finding out his mother is a cheater (which, again, she had the right to be).