Technically, we don't know that he is a Mistborn, he has a bead of Lerasium but per WoB he has not used it. We also know from WoB that he used Allomancy in the Shallan flashback but for all we know he is just a misting (soother/rioter most likely). We do know for sure that he has access to Feruchemy via WoB again though, he could simply be a ferring. So he is at least twinborn but not necessarily Fullborn.
Releveant WoB:
Feruchemy:
Lerasium: