I would get some Maracyn.
Not only will that treat the potential fungus he has, but it should help with the infection he probably has going on which is causing his swimming problems. Dosing it might be kinda tricky - you'll have to figure out how much water he's in to know dosage. The problem with it is that the tablets come in dosage sizes for 10 gallons. Cutting the tablets down isn't that hard, but you might have to crush one of them up and divide it up with a razor blade or something. I can help you out with the maths on how to get it divided, etc. One other thing - if you can hit a drug store or some store that sells those syringe things (not the kind w/needles but the kind that you dispense liquid from) get one. It will make your life MUCH easier. You can also get an eye dropper kind of thing for dispensing medicines to little kids, too. Make sure it measures ml.
I can help you figure out how to do it all later on when you get back from work.
You will only need to get the smallest amount of medication - I think it's an 8 pack.