I'm new to the fish keeping. But, if when in doubt as for all my research that I have done. Epsom salt works miracles. 1 tablespoon per gallon for 10 to 15 minute. I would try that first, if it worsens I would try after Fritz Aquatics Mardel Maracyn or API E.M. Erythromycin Freshwater... I hope your Betta well.
Hopefully with someone with more experience will respond.