The best way to treat fin rot is with warm, clean water.
I agree with Matt's water change schedule. In a one gallon tank, especially with fin rot issues, I would do a minimum of two 50% water changes and one 100% water change a week. If you want to do more frequent 50% changes, that's fine - but I'd still do the occasional 100% just to really get the unfiltered 1 gal tank clean.
Do you already have the larger tank? If you do, and you're currently cycling it, you could heat it, and float his 1 gal tank inside. Some people have suggested clipping the smaller tank to the larger one with something like clothespins.
You didn't say whether you're using the Omega One Betta Buffet pellets or flakes. I prefer the pellets. They're easier to measure out, and they make less of a mess. Since water quality is important in treating fin rot, the less leftover food, the better your water quality.
Is he still swimming sideways and floating? If so, well, a higher temp will help. But you can also add Epsom salt if he's still having swim problems. Let us know if that's still an issue, and we'll give you instructions on how to use it.