What is happening to the food he Doesn't eat??? Is it staying in the tank, where it will rot, and quickly foul the water. Foul water makes a stressed, sick fish. What IS the temperature of the water..........the warmer the better. When did you last change some water? Frequent small water changes, siphoning off waste from the tank bottom, are best. I dont' know where you are located but you can try putting him in a large container outside for a couple weeks (or more if you are in a warm climate). Right now it is in the 80s in NJ and they look, do great outside. A big container (5-gallon bucket, never used for detergents works great!), some water plants, and the natural sunlight and insects they get will make them very happy!). I would get a clean container. Add some clean water plus some water let it sit a day or two, add some water plants, than add your betta, get him outside in sunlight. Feed him some live food if you can get it. Mosquito larvae, white worms, small earthworms, pet stores often sell wingless fruit fly cultures which are also a great betta food.

Whatever you do, don't do a full water change. Have you checked the parameters of the aquarium using a liquid test? Strip tests are fake. Other than not eating, is he showing symptoms of bloating or breathing hard? Is his stomach area very big?