Well, in some versions, Percy might be initially shocked. He could be like 'Whoa, I had no idea!'. Then he might start to see Nico in a new light, with more understanding and compassion.
In certain fanfictions, Percy's reaction could be a bit more complex. He might be conflicted because he has his own pre - existing relationships and feelings. He could struggle with whether he should encourage Nico's feelings or gently let him down. He might also worry about how their mutual friends would react, so he takes some time to think about it all before doing anything.
Nico might be initially shocked. He could feel a deep sense of anger towards whoever was responsible for Percy's abuse. Then he would probably become very protective of Percy, vowing to keep him safe from any further harm.
One common plot twist is that Percy actually realizes he has feelings for Nico too, but he's been suppressing them. It's like a sudden realization that hits him when he finds out Nico likes him.
Another reaction could be that Percy is worried about Sally getting into trouble because of him. He might try to convince her to stay out of it, fearing that the abuser or those associated with the abuser could harm Sally. But at the same time, deep down, he wants her support and protection.
Poseidon might be extremely angry. He could immediately seek revenge on the abuser. After all, Percy is his son and he has the power to protect him.
She would probably be furious. Artemis has a strong sense of justice and protecting the vulnerable. So, she would likely immediately start to plan how to deal with those responsible for Percy's abuse.
Paul could react with disbelief. He might think it's all some sort of elaborate joke or Percy's wild imagination. After all, the idea of demigods and Greek gods being real is quite far - fetched for a normal mortal like him.
She might be shocked and feel a great deal of sympathy for Percy. She could immediately try to comfort him and assure him that he's safe now.
I don't think it's a very common or popular theme. The Percy Jackson fandom usually focuses more on positive relationships and adventures among the characters. Choking is a rather dark and extreme concept that doesn't really fit the general tone of the fandom most of the time.
In some fanfictions, Artemis might react with intense anger. She is a protector, especially of the innocent. So she could immediately seek justice for Percy, using her power and influence among the gods to find the perpetrator.
One possible fanfiction could be 'The Unseen Pain'. In this story, Percy might be crying due to the overwhelming pressure of his demigod duties. Nico stumbles upon him in a secluded corner of Camp Half - Blood. Nico, initially hesitant, decides to approach Percy and offer his silent support.