Thanks for your advices. I’ll make sure to provide him with the vitamins and bathing treats.It is hard to tell the underlying cause of the loss of range movement in her neck without a thorough physical exam, and radiograph. I can speak from the experience saying sometimes abnormalities that occurred during the hatch, can oftentimes appear unnoticeable until the bird ages in which then, there is more stress on the deformed ligaments, and bones.
The problem may have originated during hatch where there was a great muscle pull of the neck, there may have been an injury at some time in his life that you were unaware of, or as @Miss Lydia suggested it could be a genetic deformity like scoliosis. There may very well be a misalignment in his lower cervical vertebrae, but without a radiograph, it's had to tell. Nutritional and any other condition that may affect neurological function are all possibilities.
Whether the condition be, sadly, with older birds the prognosis is less favorable as the muscles tend to become more set, and hard in place to correct as easily. That doesn't mean you can't provide the bird with a few vitamins for neurological health, like vitamin E, and thiamine (B1), just don't expect a full range of movement in his neck thereafter.
Looking at your video, the duck does seem to do well regardless, so I would focus, as of now, just making sure he's on a good diet and getting bathing time each day. Throwing scattered treats into the water while he's bathing, may improve the way his necks moves by exercise.