I'm so sorry, but you cant cure dropsy. It is caused by kidney failure (one of my bettas has it right now) and they wont live. Adding salt can help prolong there life, but sometimes it is more humane to euthanize. Take him out of the continener, it will stress him, I never lowered the water for the fish with dropsy I had, because a) they died before I could or b) were goldfish and didnt need to breathe from the surface. Change the water often (I dont know your tank size) to try to prolong his life.
And great job on keeping him this long!! I feel pathetic now (in a good way :) )