If I were you i would treat him with a melafix medication. it is realy good stuff. you should be able to get it at any local petstore or walmart. use it to treat any fungal diseases/growths. he might have tail/fin rot.. i see it alot in bettas at my work, saddly enough. the fins will deteriorate till they are grossley infested with a fluffy, stringy, or a mossy look to them. Eventually if left untreated for long periods of time the fish will become lethargic, and will starve, or die from the fungus.

pluse melafix smells lovely