Id QT him into a clean environment. My Lt. Tang had the same thing going on when I switched tanks. He was eating, but looked like a pokadotted fish. I moved him to the QT for about a week, and hes now back in the display as healthy as ever. Didnt even have to use any chemicals.
Also, while i had him in QT I raised the temp ever so slightly. I have not dealt with ich too much in saltwater, but it runs rampent in freshwater tanks and Ive dealt with it alot there. Ive learned that the "spores" spread under certain conditions and thrive in waters at about 78 degrees, raise mthe temp to 81 and the spores die off. IM not sure if this is true with saltwater ich, but it seemed to work for the Lt. Tang.