As far as we know, Stephenie has not released an actual middle name for him.
Jacob isn't an actual character he is the person in the quote from the bible from Rebbecca Jacob and Esau's mother
He is Quileute
Jacob's Jewish name in "Jacob's Rescue" was Yakov Zuckerman.
Jacob Ewaniuk's birth name is Jacob Robert Ewaniuk.
Jacob Arabo's birth name is Jacob Arabo.
Jacob Trier's birth name is Jacob Trier.
Jacob Teixeira's birth name is Jacob Teixeira.
Jacob Moushey's birth name is Jacob Moushey.
Jacob Javits's birth name is Jacob Koppel Javits.
Jacob Scipio's birth name is Jacob Muntaz Scipio.
Jacob Krarup's birth name is Jacob Oliver Krarup.