Assuming the fish is basically undamaged, he's likely spooked. Give him time to settle back down. Keep the lights low, avoid disturbances (e.g., by not moving the rocks or ornaments about) and just wait. Big fish can take weeks to settle down. If he isn't eating -- no problem. Big fish can also go for weeks without food!
It's always a good idea to treat for Finrot/Fungus after these sorts of events; even something preventative like Melafix would be useful. Some folks add small amounts of salt for the same reason, though I'm a little less convinced about the efficacy of salt in preventing bacterial infections at least. Whilst healthy fish in clean aquaria can recover from significant damage without problems (even without treatment) if fish aren't eating or are otherwise stressed, their immune system weakens and the Finrot/Fungus pathogens can get established. Once that happens, you have a problem.
Avoid "feeder fish". There's no need to use them with this species, and unless you're rearing your own livebearers for this knifefish, all you're doing is increasing the likelihood of secondary infections. Store-bought feeder fish are bred to a price, not a standard, and are maintained in abysmal conditions.
For knifefish, things like small earthworms are absolutely ideal live foods. Safe, full of nutrition, and easy to obtain. Once settled in, they should eat just about any meaty frozen or fresh food.
Do also remember these fish are nocturnal and never move about much during the day. Offer food at night, with the lights out. I'd start by "starving" him for 3-4 days, and then add one small earthworm at lights-out. Check to see if its gone the next morning; if not, remove to prevent water pollution problems.
Cheers, Neale
