are you sure he has dropsy?...
are his scales all sticking out like a pine cone?...
bettas commonly get a thing called 'bloat' they are greedy and will sometimes literally eat till they die, especially in a community type environment...sometimes the swelling of 'bloat', under the scales makes them stick out slightly almost as if they have dropsy ...i have a feeling that it isnt the medication fixing him but the (diet/lack of food) monitor his feeding...
just my thoughts...
i could easily be wrong, but i usually tend to lean towards the more common option...