First Europe Trip - Mellow Beach Destinations?

Amalfi Coast would tick all of your boxes especially if you stay in Amalfi.

I would recommend the island of Naxos in Greece. You can fly into Athens and then, depending on flight timing, either fly directly out to Naxos or stay overnight nearby and fly to Naxos the next day. The Avra Hotel in Rafina has a regular free shuttle to and from the airport and Rafina has several nice restaurants. The flight to Naxos is short (less than an hour).

On Naxos, stay at St. George Beach. The earlier in summer you go, the less crowded it will be. The beach is long and sandy, and the water stays shallow a long way out so your son can wade into it slowly. We stayed at the Studios Kalergis, right on the sand. If you get a ground floor room, you can walk from your private patio right onto the beach. They don't serve breakfast but there are nearby restaurants you can walk to. And each room has a small kitchenette, so you can pick up supplies at nearby markets. Not sure if they have family rooms but you could get two adjacent rooms.

Naxos has lots to do and see. You'd probably want to rent a car for a day or two and explore the island.

At the end of your vacation, you'll want to return to Athens at least one day prior to your flight home in case of airplane or ferry problems. If you decide on Greece, post again for hotel suggestions, etc. in Athens.

The only problem with beach holidays in Europe in the summer is that most of Europe will be on beach holidays as well.

Have you considered Menorca? It is the quietest of the Balearic Islands and is often overlooked by tourists.

https://www.cntraveller.com/gallery/best-things-to-do-in-menorca

https://www.cntraveller.com/gallery/best-beaches-in-menorca

You won't find direct flights from Boston but you'll find many connecting flights from other major Spanish and other European cities.
Personally I think it ticks all your boxes, quiet and uncrowded in summer, low crime, plenty of upscale hotels and resorts and not a Magaluf or San Antonio in sight.

My favorite beaches, to date, in Europe are in Costa de la Luz, Spain. It's essentially the coast from Tarifa up to the Portuguese border. There are few cities; mostly miles of sand and cliffs with just enough amenities to survive. Because it's a long trip from most of Europe it has by far the least crowds.

Most European beaches are very busy in the summer. I'm not sure the north coast of Spain, Atlantic or Mediterranean coast of France or coasts of Italy will be anxiety free?

I also went to a beach in Denmark that might fit the bill. It's north of Copenhagen, Hornbaek beach. It was reachable by train from Copenhagen to Helsingor, then smaller private train from there. All the transportation was included on my Copenhagen card. The place was family oriented and nicely laid back. The water was warmer than I expected and the weather is nice in July/August.

I personally associate quiet, upscale beach vacations with the Caribbean, not with Europe. Southern Spain can be very, very hot. And many lovely places (like Amalfi Coast or Madeira or Capri) have very limited beaches, with manly small pebbles that feel odd under foot.

I can't get inside your head, but it's hard for an outsider like me to balance your special needs son with the desires of the rest of the family. For example, will they take turns going someplace more challenging on day trips?

On the low end of expense, I found the beaches at Sitges, Spain to be so wide and flat at low tide that they were not so crowded. A higher-end beach we saw was at the Ritz-Carlton Abama, on Tenerife. (Nominally, that is a mass-tourism island.) The sand was trucked in, and it was technically public, but distant enough that it wasn't crowded. We didn't stay there particularly for beach time. Although it can be very windy and cool, the public beach in Ostende, Belgium, is incredibly deep and likely to be soothing. There are other beach towns to the west (beach tram line ... ) that are even less crowded, and less of a drinking culture at night as in Ostende. But Ostende's only an hour from Bruges by unreserved, frequent train (which visit is not suitable for your son.) And under two hours to Gent and Antwerp, for the rest of the family. We stayed at a luxury resort on Madeira that was quiet and nice, and I think they had built a sand beach. Madeira has great light-hiking ("the Llevadas") for the rest of the family, without disappearing for the entire day. Time has kind of passed Madeira by, so it may still be uncrowded. But it's mainly sheer, rocky islands, with hotel pools built with a view of the ocean and zero beach. Portugal can be tedious to get to.

Beaches in the Caribbean, and a few places in the U.S., are as good or better than most beaches in Europe. (Opinion.)

Beaches in the Caribbean, and a few places in the U.S., are as good or better than most beaches in Europe.

I read this frequently but you have to determine what you mean by "good" or "better".

Personally I find long, flat expanses of sand rather boring. The beaches I've been to in Florida or along the Mexican Gulf have not been anything special. Sand is sand and sea is sea (although some seas are warmer than others) so what attracts me more is the surrounding area, for example a quiet cove flanked by steep pine clad cliffs is far more interesting than a long stretch of sand backed by a never ending row of hotels and condominiums.