Were any scans run? Did he get an copies of the reports?
It could be a stroke, but it does seem strange that the doctor did not come out and say it... there can be degenerative diseases - but why does he not say the name? I think someone needs to go to the doctor with your brother and find out specifics like what it is, and the prognosis, as well as treatments, if any. Also, without any use, he may need PT or the arm will atrophy.
If I may ask, did something happen to the "left" side of his head? ... injury? ... stroke?