What kind of tank is he in? How often do you change his water? What temperature is the water in his tank?
If he's in something smaller than a 1 gallon, get him a bigger tank immediately. Until then, change his water 100% every 3 days to keep ammonia levels from building up and killing him. If the water is below 75 degrees, then he could be lethargic from the cold. Bettas are tropical fish and prefer their water to be between 75-78 degrees F.
If he's in a decent sized clean tank and the temp is warm then he may be constipated. In that case stop feeding him for two days then go out and buy a bag of frozen peas, defrost one pea, cut it into small peices he can swallow, feed it to him, and with any luck that will clear out his system and allow him to return to normal.