How often is he shaking his head? (Video?)
Mine shakes his head at times too - watch closely - does he shake his head when he has eaten? Could be adjusting the food in his crop.
Does he shake his head before or after he has made some of his various rooster noises? Crows, growls, etc.? Could be just a rooster thing.
Mine seems to shake his head when one of the ladies is near - he flops his wattles and comb at them
It could be something more sinister, I agree, so observing when he does this will hopefully give us more information. In the meantime, check out his ears to see if there are any mites/lice in them (check his head and neck too). Something else to check would be that his crop is emptying overnight - check it when he goes to bed (roosting) then first thing in the morning before eating/drinking. Night=Full, Morning=Empty.