I'd keep in mind that barring the sensory issues (which are actually quite typical for HG kids though by no means universal of course) he sounds like a perfect candidate for acceleration, maybe early entry into first? I'd check which school might be more receptive for that. Even a gifted program, which by statistical necessity will cater more to MG than to HG kids ( there are just so many more of them around) might not work so well for an HG kids who is old for grade.