Have you treated it for IPs? Does it come out and swim around when the lights are off? Is the tank in a high traffic area where it's being made nervious or startled? If it is the only fish in the tank it may feel insecure.
Make sure he's got nice, clean water (i.e. do a big water change) and DON'T feed him for a couple of days.
He's freaked out by the new environment and needs to settle down. Constantly trying to feed him will only result in excess food rotting and bad water quality.
Let him settle down and get a little hungry...and he'll be out.