Hi...

You can try something ... feeder guppies or ghost shrimps ... but you will have to quarantine them before introducing them as food. This will give back Jack his wild instincts and may induce him to start feeding.

I had a similar experience with my Piranhas, they stopped feeding after I changed my house. They even stopped having their favorite frozen tilapia ... I gave some feeder goldfish and they started feeding ...

Try out and do let know ...

Regards