I've always had bad luck with copperbands as I can't get them to eat. They're one of my favourite fishes and would really like to have one stable and eating healthily.
IME he will not refuse fresh (uncooked frozen) mussels open it slightly or dril a hole on on side so that he will have an advantage over other tank mates. Mussels are the most nutritious food. My CBB is already three years in my tank
Frozen is a good start, bloodworms, mysis, or a ground mix (LRS reef frenzy mine likes), but I think an important part is minimizing stress. In a BRS video with the owner of Reef Collectors, he mentions that an integral part of their CBB procedure is blacking out the sides of the tank. I've seen CBBs offered food and not even act like they were hunting for it (often nose down and scanning the substrate/rocks), and while there may be some that are already too far gone to start eating, I think a lot in that state just are too nervous about their environment to start thinking about food.
That said, if you buy one that's been through a full quarantine, they have to be eating prepared foods. There's really no other way to do it.