Okay, my 7-year-old daughter helped me catch him and inspect him. What we found is very small whitish colored bugs living at the base of his feathers. I did not see any eggs; his ears look fine. I couldn't find anything at the vent.
What are the very small bugs at the base of the feathers? Could those be causing the problem? There weren't tons of them, but we did see a few the more we dug around looking at his feathers.
Yes, they could be causing the problem. Maybe they are biting him and it's driving him batty. That would explain the jerking motions. I would treat him as @MrsBrooke stated in the earlier post. I think the white at the base of the feathers is either lice, or eggs from mites. A mite will lay the eggs along the base of feathers. This indicates an infestation. Mites do not always appear to the human eye. And they hide during the day. Waiting for the chicken to go to roost at night and then having dinner on your chicken. Nasty, I know. You need to treat the entire coop also.