Thanks for posting this question in HCM
Let me tell you that demetia has so many causes.
If the symtoms have been noticed for the last few months only and he is rapidly progressing, a pssibility of rapidly progessing dementia
will be likely. Other conditions that can behave in this way are autoimmune encephalitis
(more so when the GAD 65 IS ELEVATED), vitamin B12
deficinecy, thyroid problem, Hashimotos' encephalopathy
. In the background hisotory of cancer, i will advise you to discuss the MRI and all the scans with a neuroradiologist.
Please note that another rare cause of this type of illness is cruzfeldt jacob's disease
He needs to br re-evaluated by a neurologist
or cognitive expert who will synthesize all the reports and guide you. If found to have auto-immune encephalitis, he may recover with immunomodulator.
Hope this clarifies