does he look any different than he usually does? any white spots or ragged fins? Bettas often get a disease called fin rot, where the fins get ragged and look like they are rotting away. Does he have a heater? Bettas require water no lower in tempeture than 75 F. Is it a power filter? Bettas do not like fast moving water and it can stress them out causing their immune system to lower considerably allowing them to catch deseases.
I willing to bet it is the chlorine in the water. try getting some water conditioner and see if it perks him up. If you tell the person at the pet store that you need water conditioner for tap water for your fish they should be able to give you exactly what you need.
Jackie and her little babies.
Candi- Italian Greyhound
Cash- Italian Greyhound
Jasmine- Tabby cat