camp etiquette

If it's a "little more" than generic I wouldn't worry about responding.....coaches know when they send out those kinds of emails that they will likely not get responses unless it's a yes.  I  don't think you'll have any issues if you just ignore it.  If it was a personal email, then yes, a note just  nicely explaining that you don't feel like the school is a fit would probably be a good idea, but in this case, it's not needed.

I agree regarding the invite - no need to reply.  However, it would be a nice gesture for him to reach out to the coach that did some work with him and thank him.  If any return dialog comes with reference to the camp, he should just gratefully reply that the school doesn't fit the academic profile he is looking for.

You are absolutely right about the small world and it is worth while to take the time to do those little things the right way.

 My son replied to almost every contact he had.  When it came to camp invites if he had previous contact with the school he would reply. In a situation similar to yours he would send along a thanks for working with him at the camp but politely let them know he was unable to attend camp. You never know where this will lead. In some cases they would not respond.  In others it would lead to a longer discussion with the school letting him know they were interested in him. 

At at this point with your son being a 2020 I would not close any doors. He is going to change his mind about 200 more times before he settles on his final school. 

My son responded to pretty much all of these invites, even if just with a note saying "thanks for the invitation. Your camp falls during my high school season, so I probably won't be able to attend, but I'm interested in learning more about your school."

Or there was the ever popular — "thank you for the invitation. I'll check with my parents and see how it fits into our schedule. In the meantime, here's a link to my video if you'd like to take a look and let me know your thoughts on how I can improve and where I might fit into your program."

As someone once told me — you never know who your boss might be. Best to treat everyone like it might be them.