I through the Prince of Persia is good because he might get some new story line, rather sticking to just shooting sort of games. He will get some new experience.
Left of deaf is good, since i know that he hasn't played that game and FPS as well, which I know he would like it!
He already has Gears of War 2. We both were playing it last time, when i had been to his place. He seems to enjoy it very much.
OK Left 4 Dead then, yayyy ;)