I found Aleks to be *straight math*, no problem solving at levels 3-5. She was tired of the format, so never looked at anything past five, however, I'm sure she would revisit Aleks in the future.

Have you looked at Zaccaro's books, great for kids that love problem solving:

http://search.barnesandnoble.com/Challenge-Math/Edward-Zaccaro/e/9780967991559

My DD9 seems just the opposite of your son, currently doesn't like to think! Kidding aside she just really likes patterns/numbers and is really interested in learning new math concepts, she always wants to forge ahead.

It seems like a lot of real mathy kids love the problem solving, though.