C.L.
I agree to definitely talk to the counselor about it in advance. Is he sleeping in a sleeping bag? What about some kind of waterproof liner or something he could discreetly replace and take care of by himself? It's not generally recommended to have kids avoid drinking in the evening, but maybe just for this week he could be careful about how much and what he drinks after dinner time. I know there are bed liners that people use when sleeping in hotels; I believe there are sleeping bag liners or something also. A supply of liners and plastic bags or help washing them.