I just recently had a thread up about this. I seen a Desjardini at my LFS that had Hole in the Head 10x worse than your guy. He was kept in a crappy tank with no live rock or proper nutrition he needed so i scooped him up in hopes of saving him. Sure enough 2 weeks later he is almost 100% and all his color came back and the lines are almost completly gone. Im sure it was a mixture of what i was feeding, my good quality water and huge reef for him to graze on. I would definitly try something different in his diet and stay away from the garbage food with no value like brine. I always give my tangs the Seaweed with Garlic and clip it to the glass. Try out something like that.