I think your son will be fine. I have no idea what "doublespeed" 7/8 math would be - is that school lingo for Pre-Algebra? Accelerated math in most districts near here is Pre-Algebra in 6th, Algebra I in 7th, so I think that kids like your son would do just fine in Algebra I. When middle kid was accelerated in this manner, she and others who placed into this track had some gaps, but most handled the gaps easily (kids who pick up things very quickly tend to fill in gaps quickly).

I guess that most would call Algebra I in 7th rigorous. However, it sounds like the type of challenge he needs. Teachers worry about all sorts of things. However, they usually don't know your kid's potential as well as you do.