Rome + Venice or Naples

What does he say? If I were so lucky to be taken to Europe, you can bet I'd have opinions!
Two full days would be very tight for the coast.
Naples also has a high speed train, no need/use for a car.

Sure, ask your son but I'd probably choose Venice from an efficiency standpoint given the limited amount of nights you have to work with. Transport to, from and around the Amalfi Coast eats time, and there's nothing efficient about it during high season when the buses, ferries and limited local commuter trains are running to capacity. Also, you really do not want a car on the coast: it's more detriment than convenience given slow, log-jam traffic, and scarce and expensive parking. They're also looking at odd/even restrictions on driving the coast at all on June weekends, depending on your license #. Most of us would also advise against driving in Naples. We haven't done so but we've gotten a firsthand look at it from a cab and, well, I'd chew an arm off before taking THAT chaos on.

https://www.capri.com/en/e/useful-info#:~:text=A%20system%20of%20alternating%20license,2024%20on%20the%20Amalfi%20Coast.&text=Initially%20the%20system%20will%20be,April%2024th%20to%20May%202nd).

Yes, just two full days would be pretty rushed for the coast.

Venice: You can get on a 'fast' train at Venezia S. Lucia, on the island, take it directly to Rome, and pop out at Roma Termini right in the center of the city. Other than to Salerno, you can't do that on the Amalfi Coast. A morning flight from Naples usually involves staying in city versus the coast the night before as the risk of a transport snag is a bit too high for many of us.

I might also suggest flying IN to Venice and OUT of Rome unless you can find a flight out of Venice that doesn't depart early, meaning getting up at O-dark-thirty and having get to the airport before less expensive public transport choices are operating.

I saw Venice at 18 with a group of roughly the same age (none of us looking for romance) and we thought it was plenty interesting!

For the reasons given above, I'd lean toward Venice unless your son expresses a preference for heading south after understanding the significant transportation challenges if the Amalfi Coast is part of the picture. Naples with a trip to Pompeii shouldn't be a problem, other than their being a lot more to see in the area than you'd have time for.

I want to comment, though, on the idea that Naples is crowded and busy. It's a large city in a country where people live a lot of their lives outdoors. Sidewalks on the main streets in the center of the city will be crowded for sure, but the folks you see will mostly be locals or at least other Italians. Naples is in no way as touristy as Rome and Venice (both of which I love). What it shares with Rome is being a large, densely populated city. In that sense, Venice would be more of a change from Rome as long as you don't spend too much of your time around San Marco and the Rialto Bridge.

Venice is so unique. Take your son to Venice.

Another vote for Venice. A most unique city.

I hate to pile on, yet that's what I'm doing. Take everything Kathy said to heart. Venice is great for single people, too ; it's not just for couples. And it makes a lot of sense to fly into Venice and out of Rome on a multi city ticket.