After he reach certain length, I change his diet like most of my fish. Usually after reach 6-7", I give them some tetra-looking-bait fish (don't know the species). After reach 1', I give em farmed clarias spp. After reach 2', change to mackerel.... or transfer a pathogen to your fila that will kill it faster.
Some experts like fugupuff differ on the topic of feeding goldfish, but I'd never do that.
First off, your fila looks thick and healthy and could easily fast for 1-2 months no problem. But it would not have taken that long for it to take fish or cut fish, or other frozen aquatic cuisine or even pellets.
They are not fussy but they take rehoming hard and take time after that to start feeding. They also stress a bit easier than most usual fish. I think in 1-2 weeks, it would have taken what you are offering.
Please, by all means, consider not feeding your fish live feeders.
But since you said that gold fish brings diseases, I think I can give him some chopped fish perhaps or prawns.