Try every other day with the water changes and just try and be gentle. You could try a broad spectrum anti-biotic, but he might be too far gone.
Sorry your first Betta experience has to be such a difficult one. They actually are wonderful fish with lots of personality. Since u have gone so far to try and learn to care for one, I hope u consider having another if this poor guy doesn't make it. You can cycle the tank and have everything ready ahead of time. Now that u know what goes into their care, u would make a good Betta mom, and u only have to walk into most places that sell Bettas to know we really need those. If they are kept well, Bettas can live for years and they are pretty hardy in general, if they start out healthy.
I know u are really trying to save your fish and I hope it works out for u.