Try baffling his filter.
Grab a bag of silk plants from the dollar store and make his tank into a frikkin' jungle. He's probably just stressed from being 'overexposed'. Wild bettas come from what is practically underwater lawns! They really like dense planting.
If the plant is fake, turn the light on only for observation. If the plant is real, keep the light on for only about 5 hours per day. Bettas are darkwater fish and like their shade. Bright lights make them a little flaked out.
If you cycle your tank, you should be able to get away with changing perhaps 20% of his water 3 times per week or 30% of his water 2 times per week. This is less stressful than 100% water changes.
Baffle your filter. If you show me the model I'll tell you how. Your betta's unusually large fins catch filter currents like sails, blowing them around the tank exactly like a stagnant rice paddy wouldn't.