Perhaps consider getting some fish that will eat shrimp/krill or pellets. When my Silver Arowana was alone in the tank he would only take silversides... but over the last month and a half I've added 6 fish that eat krill and pellets.
A couple weeks after getting the fish he started accepting krill, which he use to spit out...and I didn't even have to starve him or cease feeding silversides. He's more aggressive/competitive now when it comes to feeding(even though the other fish don't bother with his food)and his stubbornness is fading away. Recently I got him to accept shrimp and tilapia the FIRST night I exposed him to it, he ate enough to put a huge lump in his belly.
Good luck to ya