perhaps try feeding with garlic. its only a suggestion, but garlic has been shown to improve the feedign response in fish. it might make him come out and eat a bit more. i still think hes just getting adjusted to the tank. you might try leaving the lights off for a couple days as well. i always do this when i introduce a new fish/coral anythign into the tank, it gives em a shot at finding a nice hiding spot!
good luck
jon