Poseidon is quite proud of his son. He has done Olympus a great favor with his choosing of their side—they could not have won without him, a fact the god, like his brother, is loathe to admit.
He isn't surprised that Percy did not accept immortality. His child is humble, and believes that he cannot handle power.
How right you are, my boy.
At least Percy will not have to live his life.
He can live and love and breathe—as a mortal.
Mortals are lucky.
The worst happens when you look at chaos forever and can't stop it.