He might like Hands On Equations. It is an algebra with manipulatives program. There are videos of kids doing on utube if you want you son to take a peek first. Its not really that "hard" but its different to do stuff with hands and it could help get him ready for alegbra.
Also maybe read Developing Math Talent. Its got good suggestions for learning probability, problem solving and geometry for kids this age.
Chrys

Warning: sleep deprived