I'm a novice, so pleast take this with a grain of salt... Just from my readings, it sounds like the snake was starting to get better... but then with the extra feedings and soaks, maybe it's possible that extra stress on the snake has actually weakened it enough that it can't fight off the infection. It sounds like you've fed him 4 times in less than two weeks, is that correct? I think once every 5 days should be good, maybe with something larger than what he was getting before... but that just depends on the size of your snake, which I don't know. I'd say just keep the humidity up in the tank, no soaks, regular feeding schedule, keep up with the antibiotics.

Make sure you explain all of this to the vet and get their recommendations for feeding/soaks/temperature/humidity during his recovery. I'm sure if they're a reptile vet, they can recommend the ideal environment. Best of luck.