Suggestions:
Challenge Math by Edward Zaccharo (There is a Primary Grade Version you may want to start with)
Art of Problem Solving
Life of Fred Series (will seem too easy at first, but a lot of good math in there in story form, only a few practice problems per chapter)
XtraMath (website) for basic math fact fluency
A separate suggestion:
When thinking about what level is appropriate for him to start, you may want to look at the test where he tested 8th grade level and see what kind of problems there were, and what "8th grade level" actually means. (I.e. the average score an 8th grader would get on that test, or what an advanced 8th grader is doing, etc.)
Hope that makes sense. Others here can tell you a lot more, I'm sure.
