You could try switching to Prime water conditioner. You can usually pick it up at Petco. I know Prime pulls out heavy metals as well as chlorine and chloramine. If it were me, I'd tear the tank down, rinse everything, put everything back together and then try treating the water with Prime instead and leave out the ph down. In the meantime, if you still have the one gallon, you could switch him into it with betta water. Or float him in his cup with betta water as you were doing before until you're ready to re-do the tank.
I had a tank once that killed two fish in a row in a matter of a week. I don't know why. I took it apart and haven't used it since. Their symptoms were kinda similar to yours.