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Will he need to clear Customs and go through Passport control?
Yes, he'll need to go through passport control, since he'll be exiting the Schengen area in Zurich. He won't need to go through customs control if it is a connection (i.e.: both flights are on the same ticket).
Does this sound like a feasible transfer time, or does it sound like he might well miss his connection?
Tight, but probably doable. If the flights are on separate tickets and reservations he might need to go out and check in and go back in again in ZRH, which may make it significantly riskier. But if it's the same ticket and reservation, he'll be checked in all the way through to Denver from Greece, and if he misses the connection the airline will take care of rebooking him.
Upvote:1
Switzerland is member of the Schengen area, your son will need to go through the exit passport control, so the schedule is tight, but doable.
Walking distance is at teh very maximum 20 minutes (the airport is quite compact), there is a 5-10 minutes ride in a small train shuttle to connect from the main terminal to the non-Schengen flights terminal ... so if the passport control takes 20 minutes (unlikely), your son will still make it to the boarding gate (right) on time.