My DD, who cas be somewhat anxious, went to Girl Scout overnight camp (5 nights) in the summer between 1st and 2nd and she was fine - she had a great time. There are a lot of kids who get homesick, but the counselors are trained to deal with that.

Dude's idea about sleepovers is a good one. I think it also helped my DD to "practice" sleeping in her sleeping bag before the camp - but maybe they will be indoors at his camp and that won't be a factor?