If the epsom salt treatment fails, you can also treat with pure Metronidazole powder. Seachem sells it, and it's easy to find on eBay. Just mix the powder and some tank water in a bottle cap, let a pellet sit in the mixture for 20 minutes, and feed to your betta for 14 days. I have a container on hand in case my betta ever comes down with internal parasites.
If that fails as well, you can try to get your hands on levamisole, which is a livestock de-wormer. I see levamisole fish flakes also on eBay, but let's not worry about that yet.
Here's a link to the metronidazole product on eBay: