There is a vast difference in output between AP and 8th grade, which means focusing on output seems reasonable. Some students are ready for high school at 10 (or before) but to do high school in a regular classroom requires meeting the behavioral expectations as well as the work requirements.
Personally, I thought EPGY math covered the basics but wasn't very challenging. If he likes math, I'd suggest AoPS to provide some challenge and curriculum outside the basics. It's much more challenging than EPGY and offers multiple free options as well.
I also agree that middle school may have more options than you imagine. A local middle school has kids doing math through calculus and online AP courses even in 6th grade. It does get much easier after elementary.
