Wow, I am shocked that parents of a special kid would want to hide anything. My son, 13.5, does have developmental delays, speech difficulties, and his problems are visual with his mild CP. I would never leave him with anyone that didn't know him inside-out. Which is why he is always with dh or myself...period. The exceptions are his physical, occupational, and speech therapies, however, the therapist are experienced in this area. We aren't saddling our 16 yr old with this responsibility, either. He spends time with his brother, but not because he's our sitter.
I have been to five summer camps as an adult leader since it was difficult to locate a second adult. My special guy went with me and his dr mentioned his difficulties on the medical form. I mention this because I understand the scout camp environment. I would have been very upset that I was uninformed and didn't have all info on each scout. That's quite a responsibility for a week and nothing should be hidden.
If I were in your shoes, only knowing what is written here, I would talk to the SM and inform him of the info. You have done your part to see that everyone has a sucessful week. The ball is in the court of the SM and he should discuss this with the parents.