Have you ever heard of syringomyelia? This has a lot of symptoms that you are seeing - esp. the face rubbing. Here is a site that explains it much better than I could. It is very common in Cavalier king charles spanials also a toy breed and is in a number of small breeds. It usually starts to become evident between 2 and 3 years. It is treatable with medication and surgery is coming along. It is basically a malformation in the occipital bone of the skull and spinal fluid builds and causes the symptoms. Does he scratch a lot when he is wearing a collar or do you always have him in a harness?
I hope he is just allergic to the HG but this is also something to consider. I will see what other info I can get for you. Prayers are with you and your family!
information site: htttp://sm.cavaliertalk.com.
Also Cavaliertalk.com has ALOT of info.
The appointment with your neurologist is a great starting place and they could tell you if this is a possibility.