FWIW, when I purchased mine, he wouldn't eat, although he kept looking at everything I put in the tank for the other fish.
After a couple of days, I took it out and put it in the quarantine tank all by itself. It still wouldn't eat.
I fed it adult live brine shrimp that I gut loaded with spirulina for an hour and it hogged them all down in no time at all.
I kept this up until I was sure the 'band was in good shape and then started to put frozen brine shrimp in the mix as well.
I gradually put fewer live brine in and increased the frozen amounts until all he was getting was frozen brine.
Next I tried mysis but it took awhile for it to eat those but he eventually did.
I left him in the quarantine tank for about 6 weeks and then placed him in the display tank after removing the tang and placing it in the quarantine for a few days. When I put the tang back, the copperband was already confident enought to go after food even with the tang there.
As for flake food, my main food for all my tanks is the spirulina flake food and only two fish won't eat it and they are the mandarin and the copperband.
Even my cleaner wrasses hog it down.
I have since acclimated two more copperbands for others in the area who couldn't get theirs to eat and as I am the only supplier of live adult brine shrimp around, they asked me to get theirs eating as well. | 
