Hey i didn't want to read all pages again trying to remember if anyone told you how to acclimate your betta to the new water temperature when you do 100% water change.
You need to make sure new water appropriately as his. You can have him in his changing container with about 15% of the water and keep adding new water every 5 min for about 5 times. This way he will get used to the new water temperature and you not going to shock him with drastic temperature change. Always cover his container or when you take him out using a net or lure him with a cup for changing. Bettas known as a very good jumpers and there is a lot of accidents when they jump out.