M.W.
Hi Colleen,
This sounds exactly like my son, now 8. I like the other suggestion of having a camp counselor come over to him first thing - that kind of thing has always been really helpful to my son. You may also see if the counselors can help him develop a friendship with a child who is similar in temperament. I would avoid letting your son see your anxiety, which only will make him more anxious. Be firm but loving when you detach him at the start of camp. Tell him you love him and know that he'll have fun, and that you are available if he REALLY needs you, but that you'll see him at pickup. Also, make sure (as discretely as possible) that the camp is providing a well-organized environment and that adequate supervision is given, because problems there could also contribute to your son's anxiety.
It takes a while for this to wear off. My son is never going to be one of those kids who bounces into a new situation and takes over with a big smile and a loud hello, but he can comfortably walk into new situations and deal with it with confidence. The loving and assuring environment that you provide at home really will help shape this. It's kind of late, but the pre-K and K experience are also a big help. If he has good self-esteem from positive early experiences then he'll develop his own sense of independence.
Have faith in him, give him lots of love and just a *little* push and he'll be fine. Feel free to email me if you want. Good luck!
-M.