The best way is to quarantine him for a month by himself so that he is not stressed and can eat without interruption. After a month of being healthy he can move into the main tank.
I have a 20 high set up right now waiting for my Wrasses. They will stay there a month before moving to my 55
New additions can often be shy and refuse to eat for a few days. What kinds of foods are you offering and what types of fish are you keeping him with?
Tangs in general are herbivores and should be offered plenty of greens. Clip a dried seaweed sheet to the inside glass. Provided that you don't have alot of other herbivores, that should provide plenty of food for him to nibble on through the day.
As he becomes more accustomed to his new surroundings, he should become bolder about feeding on some of the other fare that you offer the rest of the fish.
