If you are sure that your water parameters are good, that there are no contaminents in the tank, and he is not doing this to himself, then this might call for stronger measures.
You may need to treat him with a broad-spectrum antibiotic like Kanamycin or others meds like Jungle Fungus Eliminator or Jungle Binox. Although Binox may be the best choice here. This is starting to be an issue that needs to be fixed as soon as possible.
Make sure that ammonia and nitrites stay at 0ppm and nitrates below 20ppm. Your pH and temperature need to remain stable as bettas can be very sensitive to changes in these.
This treatment is likely to kill your good bacteria, so you would need to turn off the filter for the duration of treatment. Also raise the heat and add aeration to the water. Don't add salt as this may interfere with the medications effectiveness.
The best solution would be a clean, bare hospital tank or globe with fresh water. This will save your tank and the other betta from the stress of treament. Keep the water warm and aerated. Medicate as directed. Watch water parameters very closely. Let us know how he is doing.