Try a mirror in the tank also,,,the yellow tang will be soooo much more concerned with another yellow(his own reflection) than the regal...move it around a lot too(every few hours)leave it in for a couple days..It does work..
Also ,,YES absolutely put him in the 55,,its called QTing and i think we would all see a lot less losses if we all did it...
Just imagine what the tang is going through to get to you ,,and he's probably at DD now in something smaller than a 55....after being shipped in a 10in. x 10in. dark box a 55 will be a nice pleasant place for him to gather himself and get food without competition
Good luck and can't wait to see a pic