I'm not sure if your FH is completely past the stage of accepting food, but if you do try to treat any food with meds such as metro. etc. I'd suggest to mix in a bit of garlic extract (e.g. Seachem Garlic Guard), which will increase the chance your fish will take and keep the food down, but again I'm not sure if it's beyond that and not even nibbling on food. Also although it's not scientifically proven, there are many who believe that garlic helps to kill some pathogens, though I'm not sure if this extends to worms and protozoa. Good luck and hope he pulls through.