What about a three gallon? If you absolutely have to keep it in the one gal
, heat it (if you aren't already) and up the water changes even more, 50% or more a day perhaps. But you (and the betta) will really be much better off in a larger tank. Three or five gallons might not seem like much more water, but its 3-5 times the waste dilution potential, 3-5 times the resistance to temperature change, etc.
As for meds, I've had success with API's pimafix anti fungal stuff. I had gone on vacation and not told my roommate what to do, and my betta was overfed with no water changes. When I got back his fins were ragged, half turned white, and there were white blotches on his body as well. Used that stuff, as well as lots of water changes, and he was almost completly blue again in a couple of weeks. I understand however that it only treats certain types of infection and some people who've used it have still lost their fish.
So, to sum up:
Larger tank, more water changes, Pimafix, heat, filtration, good luck to you.