Lori - have you tried any of the Zaccaro books? I just got "10 Things Every Scientist and Mathematician SHould Know" and DS has liked it so far. We've not done any of the math in it just reading the stories. It's all stories about things that have gone wrong (Challenger disaster, Destruction of the Mars Orbitor, a bridge collapse etc" due to errors in math or science. At the end of each chapter, there are 4 levels of math problems with answers in the back.
The other book is CHallenge Word problems (two levels, one for PRimary and the other for middle-high) and it too has 4 levels of problems (final level being Einstein Level) which your son might like to work through on his own to see which levels he can do. The stories that go with them are cute. There is a website where you can peek inside the books.
