This really sounds like a vestibular problem which can be from the inner ear or the brain.
You can read about this problem here
www.marvistavet.com/html/body_vestibular_disease.html
It usually self resolves but the dogs often end up with a permanent head tilt leaving them looking rather quizzical.
Another possibility is a neck problem. If he was trained with a choke collar as a puppy you might be seeing some result of neck trauma. Not all spinal issues show on x-ray sometimes you have to do a myelogram or other testing to see them.
For either of these you might see if something such as accupuncture might help him. A holistic vet might be able to help you there. This site lists some
http://www.holisticvetlist.com/#search
Its a thought if traditional medicine isn't helping.
Hope this helps you!
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