This is a total long shot, but a few things in your OP caught my attention, so it's worth mentioning, just incase, as high IQ is common
is your DS Hypermobile at all ?? Have a look at "The Beighton Score" & check him against that as speaking from experience it's possible to be Hypermobile & not realise as unless it's extreme, its your normal
EHlers Danlos Hypermobility could possibly answer the query over why he is different & a lot of EDS kids are also high IQ. My DD struggled more with written work at this sort of age as her hands were painful when writing & it caused headaches & more - she never really complained much back then, partly she didn't have the medical language & partly because it was her normal, but with hindsight we now know why.
She was different in that she was very motivated though , but girls are less prone to peer pressure at that age & as a result was G&T registered, but if a subject didn't interest her, or she was stressed by the teacher or kids she sat close to, she switched off & didn't bother at all & her marks were average, so the school didn't worry
She can also lose time in a "fugue state" so can I (it's hereditary) & I was a big day dreamer at school, but looking back it was my way of switching off the overwhelming noise, lights etc
Dysautonomia from POTs which is an EDS symptom, can cause AS like symptoms - DD swings from perfectly normal, to very like AS if she's in a POTs flare & struggling - textures, tastes, sounds, light etc become overwhelming & she can be highly anxious & difficult, but hides it well at school - when she's well rested etc, she's a different kid.
If this sounds possible having looked it up, shout & I can add some links for you to look at & maybe discuss with him
"DEBROSKIS SENSIBILITIES" is another thing to look into,
HTH