Lets make a list of the common stumbling blocks (mountains) of gifted children...
speech
handwriting
social skills
I have not met your son and am no expert but I suspect that the diction quality is having a much greater impact on people's interactions with him than you may be aware. Just like teachers can't seem to comprehend that a gifted child who can't output vast amounts of neat handwriting on a boring topic might in fact be the most advanced kid in the class, I suspect adults hearing an average 2yr old voice with an average 2yrold diction cannot comprehend that they might be listening to far higher content....
My 2nd child has a DYS FSIQ. Probably not achievement yet, though she may be heading there. But my third child, who may or may not be of a similar level of giftedness is the one who knocks people's socks off (age comparison). Her speech is not any better than #2s was (as I recall it) in diction, vocab, grammar, anything. But as well great diction, vocab, etc she will talk to anyone, anywhere, NON STOP. If she runs out of things to say she will ask if you like the sound of the jingle that introduces the news on the radio.... She asks the toothbrushes how they are doing today while going to the toilet if no-one is in there to talk to her. Her preschool report is one big euphemism for "OMG would someone please just shut this kid up!!!"
She's not watching or reading anything informative or educational because my poor neglected children watch too much tv and are made to major in dress up and sandpit at this age. So she's not saying anything scientific to random strangers, it's just perfectly understandable social chit chat, what she's wearing, what she's doing, what she just saw, and on and on and on in great detail and people respond because she speaks so remarkably clearly and so socially confidently.
She's also huge, so when they hear her speak like a 4yr old they probably assume she's more like 4 than 2.5 (I was getting "So she's about 3.5?" questions before she was 2).
