Does the tank have a heater, is it cold in the room? Colder than say 78F?
Is there a filter? Do you know about the nitrification system? And not to rinse the filter pad in plain tap water, or even change the filter pads?
What is your water change schedule? Do you refill with treated tap water when you have evaporation?
Do you have a test kit, and what are the water parameters/numbers before doing a pwc
? You can always save a sample to test for after you get done.
I would increase water changes. Freshen up the tank water.
Try a new food maybe.
If he is in a tank with out a filter and no flow they can lose muscle and get weaker by not swimming much.
Does he have a Betta log or leaf, or do the Vals loop around near the top of the water like little hammocks so he can lay in them?
Laying in the substrate in the bottom increases the chance of fin issues like rot. So I would get him something to lounge in near the top 1/3 of the aquarium, not IN the filter flow if there is flow.
A 3-5G tank is always nice, some Betta aren't as much into swimming as others, but they should usually swim around the tank and swim around stem plants and look around. If they have good clean water reasonably clean substrate and a cycled tank, they should be acting normal and swimming some, getting excited to see you come into the room to feed, kinda beg for a little treat.