Ok, my initial thought is ammonia poisoning if you are only changing the water when it looks dirty. Please test the water and let us know the ammonia level. If you don't have a test kit, I'd start doing water changes until you can get one. Perhaps try two 50% water changes the first day (make sure to use conditioned, dechlorinated water - Prime is a great conditioner that many here recommend - that is the same temperature as the tank and add it slowly over the course of about an hour so that he isn't shocked) and see if that helps. Bad water conditions are the cause of the vast majority of betta diseases, so improving water quality is always the first step for treatment.
Also, please fill out the additional information below so that we can better assist you:
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)?
Excuse me, but bettas can eat peas, just not all at once. The same goes for cucumber. Freeze dried blood worms are also totally safe for your betta :)
If you don't know, fish can't eat anything whole. They have to take bites, even if they don't have teeth. If prepared correctly, peas and cucumber for fish can save lives.
Bettas are Awesome change his tank water often. I believe all the ammonia in the tank is why your betta is sick.
Your information is incorrect. It's not a matter of whether vegetable matter is whole or not. Bettas are carnivores, the digestive tract is too short to process plant material. It can tear the inside of a bettas digestive tract. It is true however that there are other species of fish which can digest plant matter quite easily, but they have evolved in order to do so.
The fact that bettas cannot readily digest plant material is another reason to stay away from betta food where the main ingredients are fillers such as wheat.
The myth that you should feed bettas peas for consitpation issues came from goldfish. Goldfish can be fed peas for constipation but Bettas cannot.
If you want a natural laxative for you betta then look into frozen daphnia. It's a bit harder to find. Some people can find it at petsmart (although mine doesn't carry it), I found mine at a local fish store.