A few questions before anyone will be able to offer much help:
What size container is he in?
How much decoration is in it? Any type of cave or tunnel he can hide in?
What are the water params for ammonia, nitrite, nitrate, pH?
I know you said you didn't know the pH in the water, but the other 3 are just as important to check. Most LFS's will test water for free if you take it in, but you'll need about 1 cup full in a zip lock baggie and it will need to be tested within an hour of removing it from the bowl/tank.
Without knowing your water params, and the other info I have requested, there isn't much else anyone can offer for help or even suggestions. Those test results will tell us alot!
When you change his water, is the new water the same temp as in the bowl? You might want to consider letting it stand in a bucket in the same room for at least 1 hr before using it. One method that has helped many of my customers in the past: Buy 1 bottle of RO water, use it without water conditioner for cleaning the bowl/tank. When the RO water is gone, fill it with tap water, treat it with water conditioner (be careful not to overdose this stuff, it does funky things to the water that will harm the fish and is hard to fix). Let that bottle of water sit overnight before using it. When it's empty or near empty, rinse it out and repeat.
One last question... is there any way you can post a picture of your betta? It is not uncommon to see them come in on shipment and suffer from various fungal and bacterial infections right away at the store. Very few stores will medicate or even recognize the signs of illness, and many stores don't clean them enough to keep them healthy. It is very possible that your fish could be sick for any number of reasons, but your description of the situation is not always enough for us to help from so far away. Pictures are always a good way for us to see what is going on first hand. Some things are more obvious to us because we've seen them and dealt with them before.