Ok. If you are changing water 100%, that means you did not attempt to cycle your tank. Cycling just means you will be depending on your filter, fish, waste, good bacterial, live plants if any to help keep the water chemistry balanced and only do intermittent partial water changes (amount determined by the water parameters you get) etc, so that you don't have to change 100% every week.
Ok. Move him to a 1 gallon hospital tank for now for ease of water change etc. Something smaller may be better if you need to float the container in his tank that has the heater on it. Yes, for now just keep the temp at around 78-79.
Start on the epsom salt immediately. Doesn't matter what brand, make sure it's unscented and no color added. Should be able to find it at grocery store or pharmacy. For now, add 1 tsp of ES to 1 gal of preconditioned same temp water, dissolve, then leave your betta in there. At 12 hour, make it to 2 tsp/gal dosage and by 24 hour, should have a total of 3 tsp ES/1 gal dosage for the sake of slowly acclimating him to the max dose.
Hopefully the experts will come along soon to give advice. I am sorry I am not much help. I still suggest PM OFL or Sakura soon. Do you have easy access to antibiotics at your LFS near by? If you are going to buy epsom salt, I think it is best to have an idea if your LFS stock any of the meds: API Tetracycline, API Fungus Cure, API General Cure, API Triple Sulfa, OR Mardel’s Maracyn or Seachem Kanaplex so you will know where to go if medication is required.
Good luck and keep us posted!