It's really hard to diagnose cases of dropsy and/or bloat because they can be so many different things

.
I would try the medication Binox by Jungle. It's pretty general and cures most bacterial
and fungal infections (your betta may have an internal infection of some kind). Just be warned that the medication is strong--I usually don't start with a full dose just to make sure the fish can handle it.
Something to try is a salt bath, which will kill a lot of parasites. Freshwater aquarium salt can be used and is only a couple of dollars for a carton (even if it doesn't end up working, it's great to have around and can sometimes treat diseases like Ich without having to resort to actual medications). The exact dosages tend to vary depending on the source, but I would try starting with just a couple tablespoons per gallon of water. If that doesn't seem to be helping, you can try larger amounts.
To give a salt bath to a fish, set up a small container with clean de-chlorinated water that's the same temperature as the water in the main aquarium. Add the proper amount of salt and mix well. Put the fish in the container and leave it there for 5 minutes or so. Then remove the fish and slowly acclimate it back to its tank. You can give it a bath twice a day.
It's also possible that it's a tumor, in which case there isn't much you can do.
Good luck!