Ash would likely be extremely heartbroken. He has always had a deep connection with Pokemon and believing that Arceus, a powerful and supposedly benevolent being, has betrayed him would be a huge blow. He might become withdrawn at first, not knowing who to trust anymore.
He may start to question his entire purpose. Ash has been on a journey with the goal of becoming a Pokemon master. But if Arceus has betrayed him, he might wonder if all his efforts were in vain. He could go on a quest to find out the truth about his journey, why Arceus did what it did, and whether there is still a path for him to follow in the Pokemon world that is true and not manipulated by Arceus.
Ash could also become angry. He might feel a sense of injustice and decide to confront Arceus. He would gather all his strength and the support of his closest Pokemon friends to seek answers and revenge if necessary. He has always been a fighter, and this betrayal would fuel his determination to set things right, even if it means going against the most powerful Pokemon in existence.