The nation of his ancestors is Khazaria
If he can find it
Arthur Koestler would have agreed, his “The Thirteenth Tribe” elaborated on the Ashkenazi - Khazar theory. I think this idea gets hotly disputed but I have no opinion either way.
I read two of his other books, on his break with Communism, and found Koestler to be a good writer. ‘Arrow in the Blue’ and ‘The Invisible Writing’ IIRC.
https://www.amazon.com/Thirteenth-Tribe-Arthur-Koestler/dp/0945001428