I know very few mids/cadets at any SA who don't struggle with this issue at some point. Being honest, there are some folks for whom the decision to attend a SA and be in the military really was the wrong choice. And they end up leaving and going on to happy lives doing something else.
However, for the overwhelming majority, there are simply moments in time when -- simply put -- things suck. That same overwhelming majority typically get over it in another day or week or month. That's not to say there won't be additional bad times, but the good times and overriding goal of graduation and service predominate. And if they leave in a moment of pique -- well, they'll regret it for the rest of their lives.
As parents, my best suggestion is to listen to your child, be understanding, but also realistic. When I complained to my dad (a WW II Marine), he would make it clear that my life really wasn't THAT bad. Yeah, I was going through some tough times, but it would get better. And it did. Try not to fall into the trap of agreeing with your child that his/her lot in life is the worst, the SA sucks, everyone is out to get him/her -- b/c (1) it's not true, and (2) it's really not helpful.
During our plebe summer, the chaplain advised us not to think about our lives in terms of the next four years. He advised us to get through the next hour -- how hard can that be? And then get to the end of the day. And then through the next day. And pretty soon, we'd be graduates. It's actually true.
Hopefully, things will improve and your DS will realize this is truly where he wants to be, warts and all. If not, be sure he has a plan for his life rather than simply, "Anything is better than this place."
Some other wise words from my father: Wherever you are is the worst. Wherever you're going is the best. And wherever you came from isn't as bad as you thought it was when you were there.
Best of luck to your family!