My DS9 with SPD has this problem. We did a neuropsych eval this summer and while I don't have the results yet, this is one thing she told me after one of the sessions. He has difficulty concentrating on what someone is saying when there is a lot of noise (like an elementary school classroom). I think it is like what Zen said, he is so sensitive to everything that sometimes he can't block out the unimportant in order to focus on the important. His teacher's complain that he doesn't listen to directions, but I think I'm starting to get a better picture of what's going on (and why he had major meltdowns in first grade when his class was really loud and out of control much of the time).
FWIW, I had his hearing tested back then by an audiologist and it came back fine. I thought I was wrong, but now I think those tests just don't capture the phenomenon we are talking about here.
The neuropsych gave recommendations for school: seat him at the front near where the teacher speaks and to physically touch him on the shoulder every once in a while to "bring him back" if it seems like he has lost focus.
Perhaps, your son could be seated closer to the counselors at camp? Or mention the problem to one of them and ask them to be sure to check in with him that he heard?
