Wes, now is the time to get him introduced to high quality foods that some piranhas are finicky about and refuse to eat. Keep in mind that he may not always eat this aggressively once he puts back a healthy fat storage again, but take advantage of this time to use his hunger drive to your favor.
I already have mine eating earthworms, beefheart, catfish fillets, and smelt. I know he will have no hesitancy with feeders, since the greedy monster has already taken out all of my neons that were fast and cunning enough to survive with our old rhom and my ternetzi.
Anyway, at least get him on worms and beefheart. Some piranhas take some time getting used to those foods, and they will really help bulk him up fast.