Why does he have dropsy? Treating the cause is more likely to cure it than anything else. It's usually bad water quality or overcrowding - what are your water stats? Is the tank cycled? Dropsy is what happens when an internal bacterial infection (which sometimes does not show symptoms) is left untreated. Try antibiotics - in USA, a combination of maracyn 1 and 2 is the only thing that will really do anything but he may well die. ISOLATE HIM. Use a 5-odd gallon plastic bin with a spare heater. A filter is ideal but not vital because you can't have a biological filter if you're treating with antibiotics.
Get a photo from above if you can, that will let us make a diagnosis.