Unfortunately this is generally a sign of a very sick bird. Birds will try to hide signs of illness and when they do look sick, they have usually been sick for a few days. They have such a strong survival instinct, even a gravely ill bird will continue to eat. I would take the bird to an avian vet as soon as possible. If there is not an emergency clinic, take him first thing in the morning.
Place a lamp near the cage for added warmth and cover all but one side of the cage to keep him warm. Make sure his food and water is within easy reach. If he is sitting on the bottom of the cage, put some food and water there for him. Go ahead and leave the light on him all night to encourage him to keep eating and to keep him warm.
I hope he feels better soon,