You should NEVER do 100% water changes. I don't want to be mean, but they are B-A-D. It kills ALL the beneficial bacteria in the tank and also stresses the fish out way to much. He is showing severe signs of stress. I would try and keep things as normal and stress-free as possible. Bettas are very sensitive to water changes and a weekly 10% water change is the very maximum. Trust me on this, right now I own 21 fish, 3 aquariums, and have been keeping fish for 5+ years. He should make it through this time, but be careful in the future.
