Is the new water you put him in straight from the tap? If so, there might be chlorine or other chemicals in the water which might be "shocking" your fish, in a sense.
Even if it's not this, your fish may be just reacting from the trauma of the accident, in which case he'll probably perk up in a little while, at most a few days, I'm sure.
If you suspect it might be the chlorine, take some water in a pot and leave it out overnight for the chlorine to evaporate. Then, change your fishies water bit by bit with the dechlorinated stuff. Make sure you do it gradually to avoid shocking your fish more.
Good luck, I hope for the best with you and your Betta. :)