What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
Please answer these.
So, I assume you do not have a heater. You may want to invest in one, it will be better for your betta in every which way possible. And there are several that work great and arn't terribly expensive! :)
He may just not like his food. Next time you go to a fish store, invest in some Frozen Brine Shrimp. These are basically Super Prime Filet Minion (<--- spelled that wrong) feasts for bettas. A small chunk (like, a tiny chip off the block) is enough to usually spark your bettas appitite.