Mine is aprox. 8 years old and every bit of 15" lol His dorsal spines are over 6" long. He eats whole squid, octopus, salmon, grouper and various other fresh ocean fish.
Anyway...DaneDodger said, it's not good to feed the freshwater fish, as it WILL cause liver disease.
You can get yours to the point where he'll recognize a "feeder stick" as "it's time to eat!!" lol Every time you pick up that stick, he'll be there, begging. Try sticking the shrimp on a skewer or hold on to it with a small set of tongs. Once he's used to coming to the stick or tongs, for food, then you can try offering other frozen foods.