Important question: Did you add water conditoner before you put him in and did you accumulate him slowly to the new water??
how big is the tank?
does it have a filter?
Is there a place for him to hide?
I am thinking:
1) something is wrong with his water
2) he is new and just needs time to adjust to his new digs. Many times it takes the betta a few days or more to adjust.
Did you notice anything physically wrong, like white spots or fuzzies, ect?
If he does not improve, I would do a 100% water change, add conditioner and reintroduce him to the new water slowly. Something nasty may of gotten into his water.