What math has he actually learnt, both in school and elsewhere?
Does "tests at an 8th grade level" mean he actually knows all the school math up to 8th grade level, or does it mean he can do 3rd grade math as well as a typical (median) 8th grader? In the latter case, he still needs to learn all the grade 3-8 material, but you want a quick efficient way to do it.
As well as the other good suggestions above, there is EPGY (now called giftedandtalented.com) which has an online K-7 math course. The "Independent Study" version costs $60/quarter (=$20/month). You could play it safe and start at grade 3 if you fear gaps, (and there's some set theory and logic in there you don't see in school courses) and he could probably work through to end of grade 7 in less than a year. Make sure it's on "gifted setting". It's a bit dry but he can learn the material quickly.
Starting with algebra I (or prealgebra or earlier)
artofproblemsolving.com
is the place to go. They have books ($50ish), online classes (a few $100 for 12-25 weekly classes), videos (free) for some classes and Alcumus (a problem bank, free). It mostly starts at prealgebra level, though they are writing books for grade 2-5 but they won't all be out for a few years.
I'd recommend artofproblemsolving.com if solidly ready at at least prealgebra level, otherwise if concerned about gaps, have a quick run through EPGY.
