Help with emaciated blue tang!

[SleepyPirate]

Recently received a Blue tang for free included in a setup I bought its extremly thin to the point you can see its inner organs. The owner claimed it wouldnt eat but when I put my system together it ate for the first few days but ALWAYS laid on the rocks untill feeding then laid back down. Now its the 4th day or so and it remains hidden and wont come out to eat. What should I do its only 60gallons and if he dies it could cause some serious problems but at the same time I would like to keep him and see him live.