Aquarium salt. Dissolve 1 tsp in a bit of tank water, then add to his bowl. For the next five days, change his water (100% or all but the 15% you save) and redo the salt treatment. If there is no improvement, he may need medications.
For a two gallon, unfiltered bowl, you probably need to change the water twice a week, one 50% water change and one 100% water change. You don't need to rinse off his stones every single time you change his water if you don't want to; good bacteria will grow on the stones and plants and will help to keep the ammonia down.
75 degrees is a little cold for a betta. You may want to get a small heater that will keep his water at 78 degrees or warmer.
Out of curiosity, why don't you use tap water and just treat it with a good water conditioner like Prime? I bet that bottled betta water gets expensive and it will get even more expensive because he really should have his water changed twice a week or his popeye will return.
I hope Blue gets better soon, keep us posted.