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But, homeschooling him, I keep thinking that most kids could move as quickly as he has if they were given half a chance.

I often think the same thing. In fact though I've read that many HSers do quickly get a grade level ahead since 1:1 instruction is so much more efficient. I read where a teacher said that the top Xth (30???) percentile of a class could work a grade level up.

So all this serves to feed into GT denial.