Like I posted, a quick salt bath with aquarium salts should destroy the living infection as it stands. It's a quick and easy alternative to relying on those other medications. Kill that which is eating him first. Like a doctor, you have to stabilize the patient before you can begin to heal him.
Do a salt wash in an unfiltered container. Then keep him in a small clean QT. I'd suggest unfiltered, since most medications work best without filtration, but I do suggest a small heater to keep him comfortable. Provide him with a dark hiding place and/or a silk plant he can find cover in. I wouldn't add any gravel, as it can house fungus and bacteria. But the glare from a plain-bottomed tank can be very stressful, so paint it black or in another dark color if you can to reduce glare. This will greatly reduce his level of stress at being moved. Probably the reason you've had trouble in the past with bettas dying in QT is because the added stress compounded with the illness pushed them to their limit.
Do salt baths and keep him in indirect, low-lighting until there is no sign of fluff or fungus. This will 'stabilize' the ongoing infection, kill the microbes and flora causing it, then you can begin healing and prevantative treatment with the other medicines reccomended. Also, begin feeding a very protein-rich diet. You can even supplement with slivers of beef heart, gelled waterfleas, or tiny bites of steamed cocktail shrimp from your local store. He'll need the extra protein-boost to regrow the lost tissue and finnage. I also suggest that if you can get your hands on almond leaves or blackwater extract to add this in small doses to his QT. It's more like their natural water conditions and he may find this soothing if done gradually.
This should ensure that your fish, if it is salvageable, will make it out alive.
