Dropsy usually shows as swelling and scales lifting out from the body. I suspect this fish has perhaps an injury to the swim bladder. The bladder is, what fish use to control their buoyancy and orientation in the water.
If his belly is a bit swollen, he might be constipated. If you can get him to eat some cooked, shelled, soft mashed pea, in small pieces [ from frozen ones], it may help get his bowel working again and that may solve the swim bladder issue too. Pressure from the gut can impact the bladder.
If he is still eating, try to get him to eat some pea. If it works, great. If not, there is not much else you can try. If the fish is not eating, you may want to consider euthanasia. It's not an easy thing but it is kinder than allowing the fish to suffer with something that won't get better. Sometimes swim bladder problems just do not get better. Swim bladder issues can also be the result of an injury, such as the fish having banged into something or another fish having banged into it.
I wish you luck.