The islands that you mention Santorini and Mykonos are those that are the most promoted by tourism marketing and that can be found in all tour operator documentations or websites. As a result, these islands are invaded by hordes of tourists and cruise passengers from June 15 to September 15.
That being said, you have every right to like it.
If Santorini is undoubtedly a "must see", know that there are more than 200 inhabited and visitable islands in Greece
Mykonos is an island particularly recommended for partying, spending a lot of money and seeing celebrities or VIPs.
I'm not sure this is what you are looking for with your 14 year old son.
You should post your message in the Greece forum, but I can now, and without any crystal ball, give you a summary of what you will be told:
The other islands that will be recommended to you are also the most targeted by tourist marketing after Mykonos and Santorini.
First will come Naxos (Naxos has been particularly targeted by US travel agencies for a few years)
Crete will also be recommended to you, but you must be able to stay there at least 5 days if you want to explore the island which is very large.
Then will eventually come Milos and Paros.
For the trip, find a US-Athens flight that allows you to make a stopover in Athens and take a flight to Santorini because if you want to visit Athens you will have plenty of time to do so at the end of your stay before returning to the US.
Note that United has codeshare agreements with Aegean/Olympics, the Greek airlines. It is possible to have a US-Santorini with a stopover in Athens with a single ticket.
To visit the basics of Athens, the archaeological sites and museums and two or three other things you have to plan to stay 3 nights.
As indicated previously reserve these three nights for the end of your stay.
Check out these websites:
For the Cyclades islands:
https://www.e-kyklades.gr/travel/?lang=en
For Athens:
https://www.thisisathens.org/