The University of Virginia (which has an extremely strong tennis program) has a popular sleep away camp in the summer (search for the University of Virginia Nike Tennis Camp).
If your kid is very strong, the USTA training center at College Park (
www.jtcc.org) has big summer day camp program that offers world class instruction.
I have also heard very nice things about the summer camp (run by 4 Star Tennis) at Bullis high school in Potomac, MD.