It could still be an internal thing so try and get a wide-range anti-bacterial med. pima fix is very strong for bettas but it might work if diluted more than the average dosage, about half or what it recommends.
Keep adding salt, about half a teaspoon per gallon.
Keep him in the dark but keep the temp around 80f
try that and stay updating me/us!

I know you can get PimaFix at Wal-Mart by me! Good luck once again Gina!