Get someone to help you hold him...get a flashlight and very gently open his beak and check out the inside of his mouth and down his throat. The insides should look clean and normal. If you see yellowish or whitish spots or lesions, that is Canker. It too is deadly but it needs a separate treatment. They can get canker from drinking contaminated water..out of rain puddles, etc. That might be why he's shaking his head. Also, check out his ears while you're there.