Was in Santorini and Athens (along with Crete) for my honeymoon 4 years ago.
You've been to Santorini, so this is probably stuff you already know, but - evening meal and sunset in Oia is fantastic.
I loved the history of the place so nerded out on some of that - but even if you're not, I feel the Akrotiri ruins is a must-visit:
https://www.tripadvisor.com/Attraction_Review-g1189834-d4126654-Reviews-Akrotiri_Archaeological_Site-Akrotiri_Santorini_Cyclades_South_Aegean.html. It's a village that was covered by volcanic ash/lava thousands of years ago. Lots of very intact houses, steps, urns, etc. and under the cover of a large roof.
I enjoyed the beaches on both sides of the island - one with red sand, the other with black. Both beaches are worth it. We enjoyed the black a bit more (
https://www.tripadvisor.com/Attraction_Review-g658914-d2663198-Reviews-Kamari_Beach-Kamari_Santorini_Cyclades_South_Aegean.html) - there were rentable chairs and umbrellas, plus the snorkeling up against the cliff was very pretty (really clear water). Buy some cheap (used) aqua sox at one of the beach-side stores. Lots of rocks in the water. I now also remember there is another site of ruins at the top of that same cliff. They were also very cool -
https://www.tripadvisor.com/Attraction_Review-g658914-d243145-Reviews-Ancient_Thera-Kamari_Santorini_Cyclades_South_Aegean.html
Your son may get more of an . . . education at the red beach.
https://www.tripadvisor.com/Attraction_Review-g1189834-d641334-Reviews-Red_Beach-Akrotiri_Santorini_Cyclades_South_Aegean.html Definitely more liberated swimwear there.
Santorini is so great, even just hiking the trail between Fira and Oia is pretty spectacular. And the museum in Thera is worth a visit.
One of the best meals we had while there was in the center of the island, away from the towns, Called Meteza Mas
https://www.tripadvisor.com/Restaurant_Review-g1097549-d1939380-Reviews-Metaxy_Mas_Tavern-Pyrgos_Santorini_Cyclades_South_Aegean.html Fantastic, authentic Greek food. The views are great during the day, but even at night the ambience is fantastic. Highly recommended.
Athens - we were only there for a day and half. Loved the Acropolis and the Acropolis Museum at/near its base is very well done and worth a visit. I actually would have stayed longer there if I had a chance. Lots of museums, ancient sites, and good restaurants. Never had any problems there - criminal or otherwise.
Hope you have a great time!