Getting an Ancestry DNA test on him would find relatives that might know.I was lucky enough that my mom's cousin published an entire book on their paternal line. It was so cool! Probably not as tough to research as they come from nobility. Lots of formal records going back farther than Roman Empire. Her maternal side is Cherokee and Irish, so way less records there.
My Dad's side is a lot more difficult to pin down. Mostly because of lack of formal records, along with a common name on his maternal side, and the fact that it was kept a secret for a long time that he was adopted, and there are only spotty records about his paternal line. Apparently, when his father's family came to America, they either did not know how to spell their name, or changed the spelling.
My father knows very little about and really wants nothing to do with his bio dad, and I can't say I blame him. I've come to the realization that lack of records can sometimes be a result of illiteracy, illegitimacy, and people procreating without formal marriage. So I think I've gone about as far as I can.
I have so many cousins!