Well, for a fish to be happy and healthy, the need essential bacteria that live in their water. But every time you change the entire tank, its stripped of all these bacteria, and its very stressful for them because then all those bacteria need to start all over. Then right when these bacteria are back, you clean the tank and get rid of them all.(Sorry if that doesnt make sense, but its the best I can explain it xD) This time when you clean your tank, just take a cup and scoop out half the water... youll need to do this a few times a week if your tanks only one gallon.
This usually wont kill your fish right away, but like I said, its extremely stressfull for them. If youve been doing this for a year, all that could just be catching up with him... Or he could have something entirly different thats not related to water changing.
But either way, your fish will definatly improve when he has stable water conditions. If you can, get a tank at least 5 gallons (these are SO much easier to keep! And you only half to clean it once a month or so) and some Cycle.