try getting him on shrimp and fish fillet.. he will probably some kind of prepared food when you get him, bloodworms or krill or something.. if its bloodworms then cut small pieces of fish or shrimp and mix it in with the worms in a way that he cannot avoid getting the food you want him to eat in his mouth.. (since its soaked with bloodworms or whatever it may be it will taste the same and he will eventually eat it) make sure to ask what they have been feeding him.
pt 2
If he only takes feeders.
Starve him for a while.. (maybe a week) and then give him 1 or 2 small feeders and make sure he is not satisfied! then you will reach for another "feeder" which in reality is a piece of what you want him to eat. he will hit it like a feeder and hopefully give in.. if this does not work then keep trying until this works. and one thing, do not starve him for longer periods since dats (note this is just my own humble opinion, someone more experienced will probably correct me) tend to get sic easier on an empty stomach.
hope this will help.
