If you feel that your Betta does not like his tank, you may want to examine tanks specifics. It is important to note behavior changes, as it could be something unrelated to your tank that is an issue. It is important that if you're not running a tank filter, that you change the water on a frequent schedule.
A Betta in its natural environment is accustom to shallow water. In a tank environment, I do like to provide at least 3 gallons of water. If your using a smaller tank consider a possible upgrade. If you use a bowl, not a tank, these can distort vision for your fish. Checking the PH of the water inside your tank is important to maintain a healthy tank for your Betta as well.
Betta fish do seem to enjoy having items in the tank that they can hide in as well. You may try adding a few aquarium decorations like foliage, or a small cave.