One more idea. If you're willing to do a DNA test, that can help you identify cousins who share DNA, and from that you might be able to triangulate more about his family. I used Ancestry DNA, because then it links to my tree, which the computer cross-checks against my DNA matches to find common ancestors. My brother used 23andMe, and I know that also returns people who share DNA. However, there are no associated family trees so you would have to contact individuals personally to ask them for that information.