I recommend mysis dipped in garlic. However, there is a VERY GOOD chance that it will never eat. Sometimes these fish are better left on the reef. A good rule of thumb here is:always watch a fish eat before purchasing.
Get " LIVE " brine shrimp from LFS if you can to get it eating and then mix in mysis after it starts and then just mysis. You can try cracked open FW clams if you can get them.
if you have aiptasia in your tank your copperband is best to keep this pest under control.if you dont have any you could conceider finding some LR with some attached to entice him to eat.I fed mine emerald entrese
one thing you should definitely try first is clams or oysters. go to the grocery store and buy one. crack it open and put it in your tank. butterflies in general go crazy for them. if this doesn't help then you've got a problem.