Change planes in Zurich

Is this on one ticket or two separate tickets?

When he arrives in Zurich, he will go through Schengen immigration.

If this is on one ticket, and if he has checked bags, they should automatically be transferred to his next flight. If for some reason he misses the connection, the airline is obligated to get him to Valencia on the next available flight at no charge. (Assuming the delay was not his fault.) Ninety minutes should be enough time.

If two tickets, then he would go through immigration, collect his bags, recheck them in, go through security and then on to his gate. Ninety minutes would be tough.

At Valencia, he will not have to go through immigration but would pass through customs. This is more than likely a walk through.

Check to see if he needs any special travel documents besides his passport due to his age.

If his inbound flight is late, or he's delayed getting off the plane and thinks the connection
will be tight, tell him to not hesitate to ask for help.

Most likely he will need to take a train from the E concourse to the Schengen part of
the terminal to catch his connecting flight.

The other thing he will have to do in Zurich is register into the EES system in order
to clear immigration. Experiences with this have been all over the place in terms
of how (un)organized it is and how long it has been taking. If it's crowded and he
doesn't assert himself, he could stand there for a while.

90 minutes is actually not a short layover time. The MCT in Zürich is 30 minutes. SWISS plans their flights like that on purpose, as they are in a fierce competition with other airlines and hubs.

They will have to go through passport control in Zürich, and do the EES registration if this is their first time entering the Schengen area. Coming from the US there is no need to go through security again.

After that it is just a walk to the next gate (maximum 10 minutes) and they will keep the gate open for them if they expect them to turn up soon. Don't forget that with connections the airline knows when you arrived, and when you are supposed to leave, and the connection manager knows how busy immigration is. They will often hold a flight a reasonable amount of time rather than having to deal with the mess a misconnecting passenger causes.

Before this spring, the 90 minutes would be more than enough time for connections from international flights in Zurich but with the current EES registration situation it’s anyone’s guess. Just make sure he’s capable of asking for help if he’s hung up in a long EES/passport control line and he’s on one ticket if he misses the connection. I know that a few weeks ago we experienced Amsterdam and Berlin doing everything they could to get people thru passport control/EES to their flights. Hopefully, Zurich is the same and your son has short wait times. Know that he will have to have his picture taken, place his hand on a screen for fingerprints scan and maybe, answer where he’s headed. This took about 5 minutes for both my husband and I.

We were there on Sunday. 3 flights arrived at once. It took 45 minutes to shuffle down the escalator and up the stairs and on the slow bus to the EES processing. Then another 45 minutes in line for that although TBF once we got to the front it only took a minute or two.

Those trying to get on the train didn’t look to be doing much better. There were plenty of crowd controllers telling you where (not) to go but nobody giving any individual help. Signs appear to be limited or non existent and in German only.

YMMV of course but at the very least he needs to be clued up about exactly where to go.

Normally 90 minutes would be fine but the new EES system could cause a delay.

I would:
1) Check to see if there are any later flights from Zurich to Valencia on his travel day. If he misses the connection, he’d likely be put on one of those later flights. If he misses his connection, make sure he knows to go to a Swiss Air agent to get rebooked.

2) When he gets off the plane, he should not stop to use the restroom or buy anything at a shop. He should check the departure boards to see what gate his flight leaves from when he arrives and follow the signs.

Thanks to all who shared advice. I can report that my son had no troubles in Zurich and made it to his connecting gate in 45 minutes, plenty of time.