Tangs, more than most fish, are subject to stress. That is why people sometime think they are ick magnets...but in reality, they are just easier to stress out. It's why folks add them in groups and take special precautions when introducing to a established reef. It's also why we harp on tank size as they really like to have some room and good hidey holes. You might try building a temporary quarantine area or box in the tank and reintroduce him that way for a while. Or go for broke and add him back with at least two other tangs. Hope your tank is big for that trick to work.