Sometimes giving fruits and vegetables in their diet can also cause this. Before this problem what was his diet. How old is he? Does he sit fluffed up or sleeps? If you were giving fruits and vegetables all along then this could happen. What fruits and vegetables do you give. Give him pellets or seeds. Might help. Always have an emergency vet.I also first didn't have a vet and one of my finch passed away
Even if your usual vet is unavailable search for an emergency vet. Keep a very close eye on him. You need to watch what he is doing. Don't keep staring at him
Keep him at a distance from you so he will feel that your not looking. Don't sit simply do something and watch him at the same time. Is he tamed
Hope he gets well soon