Bear in mind that dropsy is more of a symptom than a disease. Sometimes it can result from contagious illness; other times it can be caused by severe stress (poor water quality, excessively rapid changes of water/temperature/etc, etc). As others have said, oftentimes it is not longterm curable, although sometimes you can keep the fish more-comfortable for a long while and he may look better.
Point being, a) don't get overly hopeful, and b) if this is a fish you've had for a while, try to figure out what you can do to improve their living conditions (starting with doing water-chemistry tests), or if it is a new fish bought out of the feeder tank at a pet shop, I'd suggest euthanizing it (NOT feeding it to whatever you may've bought it to feed to) and being pickier in future about where you get your feeder fish from.
Good luck,
Pat
Point being, a) don't get overly hopeful, and b) if this is a fish you've had for a while, try to figure out what you can do to improve their living conditions (starting with doing water-chemistry tests), or if it is a new fish bought out of the feeder tank at a pet shop, I'd suggest euthanizing it (NOT feeding it to whatever you may've bought it to feed to) and being pickier in future about where you get your feeder fish from.
Good luck,
Pat