Try not to cave into him wanting a feeder, it will take it longer to get him off of them. He will eat them with time, even try worms to get its mind off live foods. This is sad, I used to see this stuff all the time. Bigger tank, fresh clean water, high quality pellets is the best you can do for the fish. The disease will get better, but may always have effects from it, scars on the head or body.
Good Luck!