I'd try LRS Fish frenzy, Rod's Fish only blend, hikari mysis and PE mysis are also good, Hikari is probably my favorite mysis because its smaller, your CB might spit out PE mysis from it being too large.
A big challenge with with copperbands is they are poorly designed for eating out of the water column and we feed most of our fish that way. Even if you get it eating prepared foods it will have to compete with your faster fish in your display (copperbands are a slow moving fish and not the sharpest tools in the shed either). Their mouths are narrow and they tend to spit out food quite a bit, so it might look like its eating a lot but it isn't because the other fish are eating what it spits out. Keeping it well fed is obviously key to its health and possibly keeps it from munching on your corals.
You need to train your copperband to recognize you as a food source so it can eat like a copperband.
I would start sticking frozen food to the glass (dont let it thaw, just pull chunk out of freezer and stick it to the glass, LRS and Rods work great for this) and once it starts eating that way start feeding it with tongs and/or a turkey baster (this is where the mysis comes in). A copperband is designed for sucking food out of shells and tubes of tubeworms so a turkey baster works great in replicating this, just push the food down far enough for him to reach it out of the tube but not hard enough where it shoots all the food out.
I've had mine for 3 years and it definitely requires special care compared to the other 39 fish in my tank. Mine will eat from the water column, but I still feed with tongs or the turkey baster at least once a day.
Hope this helps!