Okay, here's what you do:
Good you already put him in a little tank by himself; this will keep his stress level down and it will keep the other fish from attacking him, since that is the nature of the beast: to pick on the weakest one.
Okay, now at Wal-Mart or target, or any pet place, there are little fish bubble filters that can range from 5 dollars to 20 dollars (Wal-Mart is the cheapest.) they are just a little motor with a clear tube and a blue nasal on the end of it. If you put one of those in the little tank with your sick fish, the bubbles will give the water more oxygen to help him recover quickly. Also, fish like to be massaged by the bubbles that come out of the blue nasal (I don't know why, I guess if feels good to them!)
I hope that this helps!