Rotting food + fish poo = Lots of ammonia. For me, i would be changing his water daily, even more so during his hard times. I also stay away from bettafix, It doesn't seem much weaker than Melafix in my experience, and in such a small tank any dosage would more than likely be an overdose.
It sounds like dropsy if his scales are sticking out :( Can you buy him a larger tank? I would do that, plus a heater, the heat increase to temperatures that he should really be at already (78-81) will help a lot.