You might call and ask a local school what's involved. They may require testing to let him in or something like that. We take the CAT or the ITBS each year, and I suspect that helps "prove" competence.
At least around here, I know a number of people whose kids have moved back and forth between HS, public school and private school, depending upon the child's needs at the time, so I know it can be done without too much hassle. Private schools might give yuo more grief than public, since the public schools pretty much have to let your child in. The private schools may not want to deal with it if they have a full student body even without your child.
