It's a good idea to use a heater even in summer. After all, summer temperatures can vary hugely, and a heater has two jobs: one to heat the water, one to make sure temperatures don't fluctuate. A stable temperature is important, as fluctuations can stress your betta, and that's the last thing he needs whilst he is sick.
If you can, use tap water with Prime rather than bottled water, as bottled water may not have all the nutrients he needs. If he does have a larger tank, I would return him to that, as the greater volume of water will give more stability in both temperature and water quality. :)