Do you have a heater? My rescue betta had swim bladder issues off and on until I took him in. Now that he's in a heated tank among other changes he's never had a problem.
If I had to make a guess I'd say that it is because at lower temps the body can not metabolize correctly, leading to constipation and consequent pressure on the swim bladder. If you don't have a heater I'd strongly suggest that you get at least a 2.5 gallon tank and heat it. I wouldn't put him in a smaller tank for treatment either, it isn't going to do him any good and water quality will just deteriorate faster, leading to more stressful water changes.
I hope he gets better soon!