How are you keeping it so hot without a heater? The bowl temp will be several degrees below room temp and a glass in tank thermometer is needed.. make sure it stays under 80F as columnaris responds to higher temperatures, especially over 80F so you don't want to jump start the infection before you have a chance to treat it.
Water change will need to be at least 100% every other day. Don't let it go more than 48 hours without a change as you could have as much as .25ppm ammonia built up by this time.
Move him away from the other fish.. don't let him see them. The stress will weaken his immune system and may even kill him in this weakened state.
Do water changes like this to avoid shock:
The 50% changes the betta can be kept in the bowl and use a turkey baster to remove half the water and as much of the debris as possible. For the 100% you need to remove him - scoop him out with a plastic solo type cup and set aside while you thoroughly rinse the bowl and gravel to remove the debris. Then he should be acclimated to the new water by floating for an hour while you slowly add a couple tablespoons of new water to the cup every 10 minutes. When you release him, try to let as little of the old cup water back into the tank as possible. All water changes should use same temp water, matched to running tap using the in tank thermometer and the water needs to be premixed with conditioner before adding it to the betta tank. If you don't already have anything, you can use gallon water jugs from the grocery store - rinsed thoroughly in hot water but no chems.
I would add epsom salt - 100% pure magnesium sulfate. It needs the ingredients label checked with no additives of any kind - it should be white and it should not smell. Predissolve 1 tsp per gallon in separate cup/container and add slowly over an hour to the 1 g QT. Once he is acclimated you dont' need to re acclimate every time you do a water change. Replace the salts only along with your water change. Leave him in this for 2 weeks along with the water changes every 48 hours.
I would also use Kanaplex, Triple Sulfa or Maracyn 1 and 2 in combo for 1-2 weeks. It will need to be wet diluted, so once you find the med(s) let us know and we can help with how to do that properly.