If you keep that filter running and do 50% weekly and 100% on week two you will eventually end up with a cycled tank anyway (so long as you only rinse the filter in tank water and don't let it dry out). It's not hard, so I do suggest looking at the sticky. http://www.bettafish.com/showthread.php?t=47838
However, 50% and 100% alternating is not enough. Sorry. You need at least two 50% during the week followed by 100% at 2 weeks end according to the mods here: http://www.bettafish.com/showthread.php?t=66595
Personally I've never had such a large tank without a filter so I can't bring any personal experience to the table, but if you want to change less often then you should use a drops test kit for ammonia daily after the one week mark and any time you see any ammonia at all it needs a change. You'll start to get a pattern and learn how long you can leave it before you need a change. And ideally, you change before you see ammonia. Like I know with my 5 gallons unfiltered they need changes every 5 days because on day six, like clockwork. I have ammonia show.
Erythromycin is a pretty good med. It might help with any external infection, but it's very unlikely to touch the belly bloat. Right now he really needs the epsoms for that bloat. You can use epsoms and the Erythromycin together.