Yeah, that happened to by sister's betta - about after month we got him.
First of all does the betta's tail look shriveled up or thinner that usual?if so The your betta has what is called fin rot, but i think you can treat it if you get meds fast enough.
Second,have you tried feeding your betta other things such as flake food and dried bloodworms (some bettas can be very picky)?
Third do you know exactly how old your betta is if he is more than six months old then that will probably be old age.
How often do you change your betta's water? Does he have a filter,heater,do you add conditioner?
How big is your tank, bowl, etc?if it is too small then your betta probably hasn,t gotten enough exersise and has become overweight.
Here's my advice - why don't you just let him die without the pain of trying so hard to stay alive (I know it is sad , but it is the kind thing to do).
P.S. As i said before that is exactly what happened to my sister's betta and passed away within maybe a week .........maybe a week and a half.........
P.S.S.This must be so hard for you , I feel so bad.