My great grandparents had another child?

Up until this point, my family was under the impression that my great-grandparents only had one child, my grandfather. However, I matched with someone who shares 6% DNA and shares matches with both sides of my grandfather's family.

I've tried to contact him directly, but he hasn't been online for nearly a year. How do I find out who this guy's parents are? I've tried looking for records under my grandparents' names, but I can't seem to find anything?