Fish that are removed from the tank for long periods of time tend to fall to the bottom of the heirarchy once they are reintroduced IME.
If you really don't think he's sick, I would try a divider made out of drop ceiling light covers or something similar and keep the Heckel partitioned off but in the same tank. Then try feeding live foods like CBW to get him going.
I would also raise the heat to 90 for a few days to speed up his metabolism.
hth
Ryan