How often do you do WC's? I'm thinking the rot's persistance could be due to a combination of cold water and possibly water quality issues. Also... please do not use pH adjusting products in the future, especially if you water has a low buffering capacity. They can create deadly fluctuations in pH that can lead to unhealthy fish - perhaps another problem here. I have over 30 bettas who are living long, healthy, productive lives... in a pH of 8.2! My eldest was 5 years old and never showed any signs of stress from the pH, and I've heard the same from many other people with overly acid and overly base water. They are adaptable fish, and it is safer to just get them used to your own water than tamper with it; a consistently inappropriate pH is healthier for them than a potentially fluctuating pH.
If your tank is 2.5g, it should be safe to heat, and with fin rot and other illnesses, warmer is generally better. 70 is below safe and much below ideal; bettas survive in temps between 72 and 84 and prefer somewhere in the 76 to 82 range. If he survives long enough to get meds, consider buying a junior heater as well - you can get 'em at WalMart for about 9 bucks.
