I read your question and I understand your concern.
To be honest I am very puzzled by your story, especially the corticobasal
ganglion degeneration (CBG) part. CBG typically presents around the age of 60. The youngest ever documented case has manifested at the age of 34 (I rechecked before answering just to be sure). So if your son had that disease at 38 with manifestations starting at 35, he would be an almost unique case in the history of the disease, I would be really careful in taking that diagnosis.
As for the distinction between MS and
stroke related to the
head trauma it should generally be easy with an
MRI if history and examination do not already make that distinction. Without knowing more about the signs and MRI report it's hard for me to say something more because these diseases can have many different presentations. The
seizure would direct more towards the head trauma rather than MS, as would the fact that the deficit has remained constant in time since the hit, without new signs, as you seem to suggest, but as I said MRI and more details are needed.
I hope things work out for the best.