They're really not difficult once they're established, and they're well worth the effort. Back in the '80s, they were pretty common, I usually had quite a few in my store at any given time, along with their relatives, the so-called "baby whales" and "dolphins".
Here's how I used to feed them: get a "betta bowl". Fill it with tank water, and put a generous amount of black worms in it. Sink it into your tank, and lay it on its side, with the lip at gravel level. That way, you can have a constant supply of live food available to him. You can easily put a few days worth of worms in there at a time - as long as your other fish are smalll and mellow, the worms that aren't immediately eaten will be fine in there for days.
I'd also turn the lights off completely until you get him fattened up. One more thing is to provide him with a "ghost tube" -a clear acrylic tube that you set into the tank for him to use as a cave. They really like to feel,secure, and this will let him feel surrounded, while allowing you to keep an eye on him without disturbing him.
Hopefully, it's not to late. Once you get him going again, they love small earthworms, brine shrimp, and eventually frozen foods. Flake/pellets are unlikely to be taken, but you never know once he's healthy.