Well, I think once you entice him to eat you can change his diet. Go to the supermarket, get some raw shrimps and chop them, if he eats that he will start eating other things afterwards. Alternatively your LFS might have the artemia.
The only other prestuff you can try is flakes but if he does not take them and you do not want to try anything else I would recommend taking the fish back to the store before he gets weaker.