Dying betta

I'm in great need of some help. I bought a male betta about 10 days ago, from the best petstore in Durban. I had him in a 2l bowl, with gravel on the bottom and a small plastic plant. A few days ago I noticed he was acting differently - less active, lying on the bottom a lot, swimming in jerky movements, etc. I'd only had him a week and I had done a partial water change so the bowl was not particularly dirty. Yesterday morning he didn't eat and I noticed his tail was suddenly looking very raggedy. As I watched him over the next few hours I saw that he was burrowing into the stones on the bottom and lying on his side, or coming to the surface and swimming into the glass, still in very jerky movements. I did a 90% water change and I called the petstore for advice, but the only thing they could recommend was using some "heal all". Which I then did. By last night he could barely get himself to the surface so I prepared more fresh water, put it in a new sterilized bowl and put him in it with more meds. I kept the water shallow so he could get air. I thought for sure he would be dead by morning but he wasn't. This morning his tail was almost all gone, but he was moving around. Slowly, but moving, and not in those same jerky movements. It seemed like he had made a turnaround. But through the day his tail has continued to disintegrate and he's very still most of the time again. I don't know what to do or what has caused this. From all my research it looks like fin rot, but he was fine 2 days ago and it has progressed so quickly, which doesn't seem to be normal for fin rot. Can anyone advise me on how to save my little friend?