George, you've just described my DD.
We don't even have testing for her. There seemed little point, since she is
so socially savvy, and has been since infancy, that there's no way that we would believe any numbers that we DID have. She has this ability to read people that is stunning even in adults-- and she's had that ability since she was a toddler. As soon as she picks up "I'm surprised" or even "not entirely comfortable here" she immediately changes her approach and makes you doubt what you've just seen.
She has done this with people trained to evaluate children.
She's functionally capable of doing things that no MG or even most HG children should be able to do, given her other profile (normative EF, more or less, etc.)-- and yet many of the people in her life NEVER see that side of her. She's not a "demonstrator" of such things. So as a parent, you'll see flashes of super-human ability and then be left wondering if you imagined it. Well, no-- I mean, there's no way that the RATE at which she learned to read was feigned. There's no faking that kind of arc.
The only times that you see that kind of thing from her, though, are when her (immature) ability to discern what is
actually expected doesn't match up with reality. So getting a stack of 25 novels for AP Literature as a 13yo-- she
assumed that she needed to have read. them. all. by the first week of class.
Being given a stack of 'background' reading of peer reviewed articles and book chapters in her first day of a STEM internship, similarly-- she thought that was "this day's reading," (not understanding that it was intended to fill the first two to three weeks, maybe be "homework" for the rest of the 10 week internship... so she finished them by the end of the afternoon. (This was ~250 pages of undergraduate-to-graduate level reading in a cross-disciplinary specialty).
She gave herself away with that one-- but only because the laboratory PI
had seen PG teens before and because she knows that they really are capable of that kind of super-human behavior, having a connection to one of the US universities that runs a large, well-known, and long-time program for such students. Most people just would not have believed it-- and would have experienced cognitive dissonance when DD proved to everyone in the lab that she
had absorbed and understood every word.
Now, people who are familiar with garden-variety gifted children? Oh,
they see all of the "tells" and then some-- so from them, once in a blue moon, we'll get that slack-jawed "I've only HEARD about _________, and now I've seen it in a real child..." when they had thought that she was "only" MG/HG. They know she is accelerated, so they do probably assume that she's HG. But many of them haven't ever seen a truly PG child. Those who have get this gleam in their eye and want to let us know what she is.
Yes, we know... 
Mostly, DD delivers
precisely what others expect and seem to want from her. Period. She doesn't draw undue attention, and she likes it that way.
She is highly motivated to make other people LIKE her, and to make them feel good. That IS her passion, much as some PG children are obsessed with math or chess. Like them, she is amazing accomplished.
She is a social prodigy. Period.
I think that you've correctly identified the largest challenge with such children. Figure out which parent/adult in your son's life has the user's manual to your child's brain and psyche-- and leverage that. In our household, it's me. Another member here, it's a father-daughter pair. That's what it was for me, too (PG dad, and EG me).
I've learned to trust my gut with my DD, because she is too able to fool everyone else. I
know when she is giving something her all (well, I have a better idea than most people, anyway). However, she is even able to fool those who have known her for many years, and those who routinely work with children, even gifted ones.
Getting adequate challenge for such children is NOT a picnic, that's for sure. Get used to being labeled a Helicopter Parent (or much worse).
Proof that we weren't just pressuring it all to happen:
her knocking back 99th percentile scores on PSAT (her first such testing experience ever), SAT, and ACT-- at 12 and 13 yo.
And no, I do
not mean 99th percentile among 12 and 13yo children. I mean among high school juniors and seniors.
She didn't know that she wasn't supposed to do that. We never let on, and so she did.
Do with that anecdote what you will.
