We had the 'exploded' thing too - aged 3 my DS now 7, couldn't ride a trike - within weeks of PT, he had accelerated faster in his gross motor skills than the therapist had ever seen and was riding without training wheels before 5. Again, aged 3, his coloring/pencil grip - nothing - now he has superb handwriting (self-taught cursive), and excellent drawing skills. It still is a 'perfectionist' issue though and he can drive us crazy with his 'I can'ts' that magically transform into accelerated stuff. Kind of getting used to it, but then there's the next new thing .... .
Anyway, to the point, your boy sounds very similar to mine in development, and I've really been there with the sleepless nights, researching etc, but yours really does sound gifted rather than ASD IMHO. My DS is totally a Math guy too, and as people on this site have re-assured me, Math excellence can be just that.
There is so much in the news at the moment about ASD (which is a good thing in many ways and many cases), that those of us with GT Math kids, almost feel that our kids are bound to be on the spectrum somewhere - as if it's a way of accounting for the ability.
Just a suggestion, maybe you could try looking at his behaviors physically/socially/emotionally without the 'blinkers' of knowing that he is so mathematical. I don't mean that to sound rude, and I find it hard to do, but taking out the 'numbers' you may see how he does socially in a different light.
