Any more specifics?
How long have you had him? What does he normally eat, and how much? Have you changed his diet, added any new fish, other changes?
Check all your tank parameters, esp salinity, and let us know.
Wait a few days, and see if he'll eat then. Don't stress him out about it, just wait a few days and try to direct food to right in front of him. Maybe try a different food at that time to see if that gets him interested in the new smell.
Also, check your params asap and see if anything is out of whack, esp salinity.