I find that if a Betta is sick (not being active) it can be a cause from over feeding, the water temperature, too much chlorine in the water, not being able to get enough oxygen, or being housed in small tanks for long periods of time. Like said above, we need more information to help you. But if you are not sure on whats wrong with your Betta (besides he is not moving around a whole lot) then my suggestion is to make sure to keep the water as clean as possible (change 25% of the water daily.). I would also suggest not feeding him for at least 3 days. (Betta's are accustomed to not eating weeks at a time) Make sure to check the water temperature at least 2 times a day. Betta
s thrive the best when water is kept between 70 to 78 degrees. (so adjust water temperature accordingly.) Other than those suggestions I am not really sure.. like I said I would need to have more information to help you further.