Be careful when changing temperatures. Too fast and you'll shock their system. you have to be slow to let them adjust.
Keep it at a constant temperature right now and lower the water level to a few inches. I would cup him in the betta cup he came in and float that in his tank.
Stop with the aquarium salt and have him on fresh water and water conditioner. Give him a day or two to rest before starting epsom salt at 1 tsp/gal (Dissolve in conditioned water first). Do this for a max of 14 days, but I suggest 7-10 days minimum. 100% daily changes to whatever water he's in, recondition it, and add in the same amount of epsom salt (Plain, no scents or colors)
Did you acclimate him to the new temperature and water source when you first got him? Also, have you rinsed everything under warm water before adding it to the tank and adding him?