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My reservation on that is that he goes so fast, and need so little consolidation, that I doubt it would help for long;

I am having this issue with my DD6. I never taught my child what a hexagon was or how to flip, slide & turn a shape, heck I did'nt even now that stuff. So she was placed in regular math this year because she had not mastered 90% of the grade1 curric going into the year.
NEways the problem is that being placed in the reg math class is slow for her, they have been slugging through 2 digit add/sub for weeks when my kid got it the first day.
I think you are right in assuming that moving him up is a short lived solution. My DD learns new stuff in school all the time, but she has the ablility to learn it faster the the rest so often becomes bored with the repetition.
I don't have the right answer but I do wish you luck!