For additional math you might consider Edward Zaccaro’s Challenge Math for the Elementary and Middle School Student (the one with the dark blue cover); his Primary Grade Challenge Math (the light blue cover) will likely be too easy. In addition to EPGY’s 7th grade, you might want to take a look at Art of Problem Solving’s Prealgebra I and II courses. They have a diagnostic test to make sure your DS is ready, and you can register your DS for the courses or let him work on his own through the books with the aid of the free online videos on the AoPS website.
