How long have you had him for?
For any other family of fish I would have said catch him (if you can) and put him in a quarantine tank, but some say Tangs suffer more by being kept in a quarantine tank then by being kept with their friends.
I'd feed him heaps of frozen or live brine shrimp and the Nori sheets.
I agree with the above about the cloudy eye - he might have banged into a coral, but if your water conditions are good, the eye will mend itself. My Coral Beauty actually had a horrible looking swollen cloudy eye a couple weeks ago, but it healed completely by itself over about five days.
If he gets really thin, I WOULD catch him and put him in a quarantine tank, cause you'd probably have nothing to lose at that stage.