If your fish is eating flakes you can feed him flake food with antibiotics in it. Be careful not to overfeed the tank as the antibiotics can effect your biological filter, but a slong as he's eating this should help.
i had the same thing with one of my clowns a few days ago. try using melafix. put the clown in a separate tank and treat for a few days. it should go away no problem. make sure you don't wait to treat. your fish could get popeye
I wouldn't treat in the main tank...go get a 10 gal tank and use the water from your main tank, powerhead and a heater. No light needed... I've used maracyn and maracyn2 with good success