Maybe he is just having a bad day..like we all have once in a while. Maybe he is lonely? Could you get him a friend possibly?
Or maybe he is bored? Does he have some toys to play with in his cage? A good thing to do is to rotate their toys once in a while.
If you can it would be good to take him out of his cage for awhile so he can streach his wings and fly around. Only do this if you can do so safely in a room with no other pets like a cat or dog. Make sure window and doors are shut also.
Another possibility, but I hope its not so is that maybe he is feeling a little under the weather..sick. Does he have any discharge around his nostrils or is he sneezing at all? Does he sit up on his perch all fluffed up?
If he is eating and drinking his water normally he should be okay. Try to give his some delicious veggie treats. Does he have a favorite?
Hope he perks up for you soon