Dying betta

I'm in need of some serious help. I have a new male betta. I've only had him about 10 days. 2 days ago he looked completely normal but I noticed that he was not acting as he usually did. He was lying on the bottom a lot. Yesterday morning he did not eat and I noticed his tail was looking raggedy. I called the petshop as soon as it opened and they recommended using Heal All, which I did. I had done a partial water change recently, but did a 90% change and added the meds. Over the course of the day I watched him deteriorate rapidly. His tail got much worse, he started burrowing between the gravel at the bottom and lying there on his side, and when he swam it was in very jerky movements and often into the glass. By last night he had almost no tail left. I prepared more fresh water, added more meds and put him in a new sterilized bowl, keeping the water shallow as he was battling to get to the surface. I was convinced he would be dead by morning but he was not. He was moving about very slowly (not just lying still like before). I thought maybe he was improving, but through the day he has gotten worse again. His tail is pretty much gone and he's not really moving anymore, except to come to the surface, where he gulps air but it comes straight out his gills. I have no idea what more to do, or why this has affected him so quickly. Please help!