My DS8 is in a similar place, he really enjoyed Hands On Equations ( http://www.borenson.com/ ) at summer camp last year. If you have an iPad, yours may like Dragon Box visual algebra ( http://www.dragonboxapp.com/ ) which wasn't that popular with DS who wants the numbers.

But for lots of reasons, starting a The Art of Problem Solving might be good: http://www.artofproblemsolving.com/