Answer:
He is the son of Poseidon.
Step-by-step explanation:
Percy was in the middle of a "battle" of capture the flag(?). He gets slashed or cut and, feeling weak, goes into the creek. The cut then heals. Then a hound suddenly appears and attacks percy. Chiron managees to kill but Percy is badly injured. Annabeth tells him to go back in to the creek and he is then healed. Above him, a glow of light appears and an image of a trident. The campers gasp and kneel before him. His father is Poseidon