There is a medication called PraziQuantal, the brand name is "Prazi Pro" that should help. Treat as directed, be careful not to overdose it. The other medication option is called Pipzine, and it can be mixed with the food and fed to the fish. Of the two, Prazi will be faster, and more effective.
I believe your fish has a tapeworm, which is not uncommon in bettas. All of the symptoms fit, and the lack of response in other treatments also fits.
Because he probably isn't eating anything at all, understandably, if he doesn't feel good, so the Prazi is the better way to treat him because it is put into the water. If there is a filter running, be sure to remove all carbon before medicating. After medicating, do a 25 - 30% water change, daily for 3 days to remove it from the water, and replace the carbon in the filter, if any.
Keep me posted, I'd like to know what happens.