How did he do in his fourth grade math acceleration this year?

Do you live in a state that has laws or regulations regarded gifted children (e.g. Pennsylvania mandates gifted IEPs)?

If they're tossing out random slippery-slope reasons, it may be that they've just made up their minds for reasons unknown to you.

(What is so horrible about accelerating other children if they need it? How is this different from letting other kids move more slowly in special ed?)