Aerate the water (bubbles) but make sure you leave the air-stone on the surface in the one gallon or it will just "blend" him.
Keep in mind the "fungus clear" is Furan-2, a wide-range antibiotic. Go ahead and use it, it won't kill your boy unless you use about 4x dose or more in bad water conditions. Furans are slower cures but they suppress growth and often block reproduction of the bacteria AND fungus. For some reason Jungle's actual fungus cure is labled "Ick Cure"...
You can break a tablet up into a drinking water bottle then use 10% of the mix from that in your one gallon container. If the bottle is kept cool and closed the remaining doses will be good for up to a week (I usually make new every 3 days). Treat for minimum 8 days external and 12 days internal infections.
I have to assume that he's got the same thing inside as he's got on his tail, so the furan should cook it all for him. You can soak any food you give him in a drop of water from the mixed bottle for 5 to 10 seconds before putting it in for him.