Columnaris infection. Try and keep him at about 74F.. lower temps for this disease help slow it down. Some say it doesn't help, many say it does. I'd lower the temp a little, if it was my fish.
Forget the bettafix. It won't help this. Here's some suggestions from Darkmoon17's treatment sticky:
Perform daily 100% water change in small tanks or ĺ water change in larger tanks. Make sure to clean the gravel. Treat with Aq.Salt: add 1 tsp/gal Aquarium Salt 3 times, 12 hours apart so that you end up with 3 times the normal concentration. Do NOT raise the temperature as it thrives in temps over 85*F, however, lowering the temperature does not seem to help fight it. Combine salt treatment with Mardelís Coppersafe, Maracyn I & II, API Erythromycin, OR API Triple Sulfa, combined with Jungleís Fungus Eliminator (if possible).
I have to be blunt and say your fish is seriously, seriously ill. Try all of the above, as he may pull through.. but yeah, he is one sick little fishy.