Put a couple (yes , 2 or 3) strips of Nori soaked in vitamins or garlic... the reason I say use a couple, is probably because your yellow tang will go nuts on one.. and the regal may be too shy to go in for some food.
I'd say if you used vitamins and garlic it would get his belly full and he'd be feeling real good.. might come around a little quicker.
To note, I own no tangs.. I have no experience.. this is just a suggestion!
Good luck, I am sure in time he'll come around
