Perhaps there's a power struggle in the story world. Percy might be seen as a threat to certain groups or individuals who then resort to abuse as a means to gain control over him. Another reason could be related to his family background. If his family has some sort of enemies, they might take out their anger on Percy.
They might be shocked at first. They could feel a sense of guilt for not noticing earlier. Maybe they immediately try to comfort Percy and offer their support, like giving him a safe place to stay and assuring him that he's not alone anymore.
The others in the seven, like Jason, would probably feel a sense of guilt for not noticing earlier. They would come together as a group to support Percy. Jason might offer his friendship and strength to Percy, telling him he doesn't have to face it alone anymore. They would also likely start to investigate who was responsible for the abuse.
It could be one of his close friends. In many fanfics, friends often play a crucial role in uncovering such secrets. They might notice Percy's unusual behavior, like being overly jumpy or withdrawn, and start to suspect something was wrong.
The gods might be outraged. They could see Percy as one of their own, especially since he has done so much for them. They might immediately seek to punish the abuser. For example, Zeus could send a bolt of lightning towards the person who abused Percy.
Well, in many such fanfics, the gods might react with anger. They could seek revenge on the abuser. For example, Zeus might send a bolt of lightning to strike the place where the abuse took place as a sign of his wrath.
Well, the gods are a diverse bunch. Apollo might use his healing powers to try and ease Percy's physical and mental pain. Artemis and her hunters could offer Percy a place among them where he would feel safe. Percy, on his part, would be in a very vulnerable state. He might be hesitant to trust anyone at first, even the gods. His friends from Camp Half - Blood would be rallying around him, providing him with love and support. They would encourage him to speak out about his experience and not let the abuser get away with it.