Each of the pearls can only be used to save one person. Thus, Percy has to decide who will be left behind. In the end, Percy chooses to leave his mother behind, remebering the prophecy, "You will fail to save what matters most in the end."
Percy's decision is difficult. He cannot leave his friends behind, even though they offer..... he doesn't want to leave his mother..... but if he makes the sacrifice to save his mother, his own quest will come to an end. I believe he made the right choice, though, I am not sure I would have done the same. The desire to save a parent would be overwhelming.