I believe he is a re-incarnation of Ebonarm or even a past dragonborn because of his ability to shout but then again he could have also been a warrior that trained with a greybeards.
He's definitely not Dragonborn, you'd absorb his soul like you do with Miraak's if he was.
Well now that my Green Shaman is complete, I'd better start cracking with this doozy.