Was the butterfly eating at the lfs? I never buy a fish from the lfs unless he is eating readily. Let someone else take the risk if they buy him before this. There will always be another fish.
As for getting him to eat now your best bet may be live brine shrimp. The trouble is if they go too long without eating their digestive systems will begin to fail, so even if they do start eating they will still starve to death. Live brine is like chocolate to a fish. Taste great, but not very good for them. Thing is to keep that digestive system open and functioning. As he get comfortable he will start eating those aptasia.