He knew of his destined job all along but he didn't want to leave the way of life he had before because he enjoyed it... when he was kicked out of the hero's party he was sad but probably understood it had something to do with his destiny... on the hike through the mountain we can see he was sad he could not longer be an adventurer but told himself "it's time to face reality" (maybe referring to knowing his destined job was not as an adventurer?). Only when he saw the people and their struggles in the village did he 'awaken' to his destiny, finally accepting it.
That's my take.