metro will be your best bet now probably- if good quality, varied foods and super clean water isnt helping then it will be worth a shot trying metro. ive read accounts of fish on the brink of death coming back when metro has been used (for hex and HITH) so its good stuff. flagyl is metro, flagyl is what they call the 'human' version of the drug, not sure what its used for but its the same thing.
have you rung yout vet? if the fish means a bit to ya it wouldnt hurt to take it down if your vet wants to see it (if hes well enough to travel), explain what it is, print out a few pages on HITH and metro usage and see if hell prescribe it then.
because the fish isnt eating it will probably be necessary to do a water treatment, there should be a few articles on that around on the net. if you can get him in a warm tank, free of substrate with nice clean water, good foods and metro it should give him a good boost.
good luck!