My DS8 had a number of things in common with your DS around that age. He was a perfectionist and while not speaking or walking late - he did wait on those until he was sure and then things came in giant leaps. I asked numerous times about autism or similar disorders. At this point, everyone has said that he is PG mathmatically (with a very strong memory and verbal IQ), but does not have any form of autism. (He may have some amount of ADD - very disorganized and forgetful with things - I don't think your son has any signs of this. I am a bit undecided on whether I agree with that diagnosis or not - want to see if anything changes in fulltime GT program next year.)
He had the phase of always watching clocks and I remember several discussions about when it is okay to add numbers to a conversation. (By the way, his math has really taken off this year - second grade - they just started letting him work online at his own pace in school in place of regular math a couple of weeks ago and now he is more than 3/4 done with pre-algebra.)
