I'd write a letter and say there is a possibly that he was your father.I did this looking for my birth mom I mistakenly wrote to her ex sister in law.This lady was so happy I was looking for my family and said she knew someday I would try to contact them.My birth mom had died but I i'm now in contact with my birth siblings.Never be afraid to ask questions.If they don't know about you or don't want to answer at least you tried.