You're not doing enough water changes. You need large weekly water changes for a tank that small, even if it is cycled, and test the ammonia on a weekly basis too because the cycles tend to break down. You also need to always use conditioner.. Prime is a good one.
His problem is ammonia/nitrite poisoning, chlorine poisoning, any maybe at this point also an internal infection. I'm surprised he's stayed well this long.. hearty little fish.
Here's some links that can help you with water changes and cycling tanks: http://www.bettafish.com/showthread.php?t=115758
I would say if you're not going to do the 100% change a week that's suggested in another sticky thread you need to test for ammonia on a weekly basis to make sure your bio filter hasn't broken down. http://www.bettafish.com/showthread.php?t=47838
At this point.. He needs a 100% water changes - acclimate him don't shock him - with a good conditioner like Prime. You'll also need to get some epsom salts 100% pure with no additives as per the ingredient label CVS is a good brand. You'll want to mix 1/4 to 1/2 tsp per gallon predissolved and add it to the tank. You can increase it after a few hours up to 1 tsp per gallon. You'll need to keep up with the water changes and redose any water you take out with more epsom salts.. like if you do a 50% put back in 50% of the epsoms you had in before. At this point he may also need an antibiotic.. Can you show photos?