While The Grayson School sounds great on paper, I would be a bit wary. They seemed very certain they would be accredited for this school year - now they are confident that it will happen in June. While I have checked into their Saturday classes for my little one, I am a bit concerned about a new school. There are so many good private schools in the area and they have proven themselves. Many have been around for over 100 years, so you know the good and the bad.
That being said, I do not know of any school that truly meets kids at their level, at least not in elementary. I keep reading about kids that already know all the K - 1st grade stuff...ummm, that is most kids around here. While there may not be very many PG kids, there are lots of MG+ kids (if you are within driving distance of Grayson).
Does your DS do summer camps? My kids have been able to pick out the gifted kids very quickly, even when they were very young. Also, I bet there are kids in your child's school (though maybe not in his classroom) who are gifted but not in an obvious way. I have written about my middle kid whom I labeled as "Not college material" when she was four. Not in college yet, but finishing up 11th grade and has a quite a list of academic achievements without trying very hard. However, after a recent awards night, people were surprised with the awards she won, and some were surprised to see her there. She just is not an obvious gifted kid.
Don't despair. Try homeschooling if that works for you. I personally would give The Grayson School a year or two to see how it goes. I am also concerned that it will be really small. That was part of the reason we pulled our kids out of a private school that had very small classes starting in middle school (elementary was fine). They could not offer the after school activities that other schools could offer, and the social circle was very limited.
Best of luck - let us know how it goes!
