It has not been revealed yet 😉

In fact, Sean’s knack is heavily touched upon in an upcoming story in the Brothers Apart series, and will be revealed then. We will learn a lot more about the littles of Trails West, and their resident cursed human when trouble comes knocking at their door.

In fact he does! Just like Sam, Dean and Jacob, the moment he was cursed a part of his mind was unlocked by that same curse! This will be a theme that is touched upon heavily in the as of yet, unnamed twelfth installment of Brothers Apart (the story I am currently agonizing over), and Sean’s ability will be revealed.