Who saves Percy's life just as he's about to be eaten by Polyphemus?

In Sea of Monsters, Percy has Polyphemus down and is about to deliver the killing blow when he realizes that he can't. He allows Polyphemus to get away, but this is a mistake as Polyphemus rounds on him and is about to eat him. This is stopped when a huge boulder hits him. It is Tyson who has finally caught up with Percy.