He could be very depressed at first. Losing such an important title that he may have worked hard for or believed in for a long time can be a huge blow to his self - esteem.
One reason could be that he fails a major quest. For example, if there's a prophecy that he must defeat a certain powerful villain to maintain his 'Chosen One' status and he doesn't succeed. Maybe the villain was too strong or Ash made a crucial mistake during the battle.
Ash would probably be initially shocked. He is used to seeing the good in people and would not expect such a betrayal from Cynthia. He might stand there in disbelief for a moment, unable to process what has just happened.
Ash would probably be very confused at first. He might try to reason with Latias, asking her why she is doing this. If she doesn't respond, he would start looking for ways to escape. He would also be worried about his Pokémon and his friends, wondering if they are safe.
Ash might be initially shocked and scared. But he's a brave trainer. He would quickly gather his courage and probably try to figure out who sent the hitman. He'd use his detective skills, which he has developed over his many adventures.
Ash would be heartbroken. He has a deep bond with Charizard, and seeing it turn against him would be like losing a part of himself. He would probably first try to reason with Charizard, thinking that there must be some mistake. When that doesn't work, he might become a bit angry, but mostly he'd be sad and determined to find out what's wrong and fix it.
Ash as the Chosen One in Pokemon fanfiction is a really interesting concept. It gives a whole new layer to his character. He could be given a special Pokemon by some mysterious force that is crucial for his destiny. This Pokemon might be one that no one has ever seen before. His friends would also play different roles, perhaps as protectors or guides to help him fulfill his chosen - one duties. And his battles would become even more epic as he has this greater responsibility on his shoulders.
His rival could be the one to betray him. Rivals are often jealous and competitive. If Ash's rival saw an opportunity to gain an advantage by betraying him, they might do it. For instance, they could lie about Ash to the Pokémon League, getting him disqualified from important battles.
Ash could get angry. He has a strong sense of justice, and he might feel that May has violated their friendship. He could challenge her, not in a violent way, but more in a way to get an explanation and to try to make her see the error of her ways. He would likely be determined to show her that what she did was wrong.
Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
Ash would be extremely devastated. Pikachu is not just his Pokémon but also his closest friend. He would likely first try to reason with Pikachu, not believing that Pikachu could truly betray him. He might be in denial for a while.