I would try it before making judgement on the program. Your son might be just fine and if he 's not, then have a plan B ready to go.
There is also the GATE Academy in Marin. It seems like a nice (private) school that is priced a little lower than most private schools.
I can also vouch for Alt School. My son currently is in a combined 2nd -3rd grade class. It is not a "gifted" school, but my son really enjoys going. He feels like he has to work hard sometimes. The style isn't for every kid as it's not rigidly structured. It is very open and about "intrinsic learning". It is new and very progressive.
