Bettas must be kept at a steady 78 degrees for the temp to be "good". What's it been at?
But anyways, to get back to the original topic if he's pineconed it looks like dropsy which is kidney failure which can be caused by a good many things. So to try to get to the bottom of this you will describe for us:
Your tank cleaning schedule
Your feeding schedule
Unfortunately there isn't much hope that he will survive which I'm sorry about but perhaps we can prevent the loss of any others and give them an increased quality of life (maybe the starting point can be getting a larger tank? We usually like to see bettas in at least 2.5 gallons).