My betta fish:he is dead...

This discussion is old but i still want to comment. Your betta suffer from a disease called dropsy. Dropsy is a common disease among fresh-water aquarium fish. The main cause is bacterial infection. Since dropsy is a symptom of an illness, it may or may not be contagious. However, it is standard practice to quarantine sick fish to prevent stress among the other fish in the tank community. This extra stress may make the others vulnerable to dropsy or other forms of disease. causative agent may be introduced through poor water quality.

did you overfeed him?

I see this is an older discussion but it sounds like your fish was constipated, no really, it can happen from over feeding. As long as the scales around his belly are not sticking out and are flat against him like normal. I don't know how useful this is now but if you have a fish that this happens too again you could try this. Take a pea, but not one that's been processed, like from a can, fresh is the best as there are not preservatives or chemicals, and put it in some water in the microwave for 25-30 seconds or so. Remove the outer shell of the pea, fish can't eat this as they can't digest it. Take a small amount of the inside of the pea mash it up a little and feed this to your fish. If he don't eat it, don't feed him for a day so he'll be hungry and try again. This will help to clean his insides, like a flush and relieve the constipation. If your fish is too constipated it can become sick and even die. You can do this with fish that are not constipated too. Once every week to help with their digestive tract. But please remember not to use anything store processed, canned, frozen, and the like because they have chemicals and preservatives that will do more harm then good to your fish. All the best. :D