If you can, look at your fish from the top. If his scales are sticking out from his body (it'll look similar to a pine cone), it is dropsy without a doubt. Unfortunately, if he does have dropsy, there is little you can do, and he is going to die. I had a molly succumb to dropsy once a few years ago and it is a pretty nasty way to go. The good news is that dropsy is usually not contagious, so you wont have to worry about other fish catching it. However, dropsy usually occurs when their is an underlying problem with the water parameters (high ammonia, nitrites, etc) so you should check all your levels and make changes accordingly.