Right...I mean, the only thing we know about Jacob for sure is that he knows (or knew) a lot about the island. Logically, I would say he was the child of a member of the Black Rock, or a direct member. If you remember his shack, I watched it again and it really looks like all the stuff in there is similar to the artifacts we saw on the Black Rock.
As far as historical figures, there have been plenty of explorers lost at sea, so perhaps he was one. I think the island "moved" in the past and appeared right on the Black Rock. It would explain it's inland location.