He has a pretty severe case of what appears to be Columnaris, which is a bacterial infection. He needs treatment with a gram-negative antibiotic, and he needs it quickly!
I would treat with API Fuan 2, Maracyn Plus
, or API Triple Sulfa. These are all available at Petsmart. If you cannot get one of these meds, get Jungle Fungus Clear from Walmart.
add an airstone. The bacteria will thrive on the added oxygen.
I would stop the Bettafix. Its use with bettas is controversial, and Columnaris requires an antibiotic.
Once you get the antibiotic, if you have questions about how to use it, let us know.