Could you give us some more information about your tank? How big is it? Is it filtered and heated? How often do you do water changes? Do you have a test kit, and if so, what are your values for pH, ammonia, nitrite and nitrate? What's the water temperature?
Since the tank has been set up for so long and the behavior change was sudden, I would guess that either your fish is in the early stages of some sort of illness or that the temperature change to cooler weather has made him lethargic. Bettas are tropical fish and need water that's 78-82 degrees in order to thrive. If your house has been getting a lot colder lately and you don't have a heater in your tank, I would invest in one.