DS (now 9) was not talking at that age (and wouldn't be for another 10 months), so a fortiori wasn't doing most things on your list. I think the interpretation "therefore, he can't be HG+" would seem perverse in the light of his later development, although he hasn't been IQ tested. So don't write off the cousin if this could be the main issue there! That said, he had been clearly understanding everything said to him and a lot that wasn't for a long time by then, and several other items on your list look familiar too. Early speech is a gift (and I'd have welcomed it!) but one can give it too much weight, I think.

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