This tang is very thin and has a severe case of fin rot. I would run him through a course of antibiotics (Furan-2) in a hospital tank, and then try to address the root of the problem. This can occur as a result of poor husbandry (diet, water quality, tankmates, etc). Diet is likely the culprit here. Tangs need seaweed, or seaweed based foods daily, and you will want to vary the menu far beyond just nori and mysis. Try to rotate his feeding schedule through different frozen foods, a good quality flake and pellet (I like hikari seaweed extreme and NLS pellets), and red/green/purple/brown nori.
Yellow tangs can bounce back from this degree if fin rot with TLC, but sometimes the fins are just too damaged and the dorsal/ventral spines won't grow back. You can still have a healthy happy fish though.