We were in much the same situation .
The school promised and it seemed to always fall apart. In 4th we started on AoPS Pre-Algebra and also hired a nice High School senior boy as a tutor 2 times a week. It was a great fit. We also started math competitions and summer math camps.
I strongly recommend these for strong math kids.
In 5th grade we switch to a private school where they started middle school and you tested into your math path for the next 4 years.
He is now in 10th grade taking AP calc bc, we will have to find a local college or online course for the next 2 years.
11 and 12 grade could prove to be interesting years.
He still does quite a few math competitions during the year and summer.
