is he in a community tank? With other fish? And Betas only need to be fed once every two days. and your supposed to only give them as much as they can eat in three minutes, cuz when they don't eat drifts to the bottom and makes the tank dirty.
Have you washed the filter properly? (As in taking it out, taking it apart, and scrubbing each bit.)
Then you might want to get one of these: [link] (Or something like it.)
You might also be over feeding him.
And there might be some funfus going on with his tail.
My best advice: Google 'Betta forum', and find a good forum with breeders and such. They'll probably get a bit testy with you for what you might have done wrong, but they have the best advice. If you really want to help your fish, they'll notice and really help you.