One possibility is drifter From Cambridge Dictionaries Online
a person who moves from one place to another or from one job to
another without any real purpose
This definition is not quite right, because it overlooks the fact that a drifter might work very hard, even though he does not accumulate much, and might have a purpose that is not recognized by conventional people as a real, valid purpose. For example, climbers often work construction jobs to earn enough money to climb. Example: Jon Krakauer, author of Into Thin Air, when he was young. His purpose was to climb, climb, climb.
More generally, some people just take longer than others to find a purpose, and the process of drifting may be necessary for them to find their purpose.
John was a drifter, but he certainly didn't fit the stereotype of a
could be used of a hard-working but aimless, or seemingly aimless, person.