He's freezing unless your room is 80F+ and then it needs to be a stable 80F+ with no dips at night peaks in the day.
What color is his poop? Is he pooping daily? Is he bloated at all?
How much stress coat are you using?
How do you acclimate during a water change?
You should get a good adjustable 25w heater like Jager or Marineland Visitherm. Always suggest giving it a good test run for 24hours in similar sized container with thermometer to make sure it will hold a proper temp. Then you'll need to acclimate your betta very slowly tot he higher temp at 1 degree per hour and no more than 5 degrees per day until you get it to 78-80 degrees. Rely on your in tank thermometer,not heater for setting.
Photos would help otherwise.
I would increase water changes to twice weekly.. add an extra 50% mid week.