I think pretty much all of us have heard “I’ve worked so hard for nothing.” Following not only a rejection but also deferrals and WL. They know they odds.
Some students may see a Wl as a win, the super high achievers do not for the most part.
There are always those super zen types (that I really admire!) who take it stride. Rare imho.
So you are not alone and neither is your son. This will pass with time like grief around a sad high school break up or dropping the big pass.
It’s emotional but not life threatening.
They’ll be ok. You’ll be ok.
Just be there and keep it light for a bit. And if they want to saddle up and relook at some schools, pull up a chair and dive in.