FWIW, we have had a Mathletics subscription via school for the past 3.5 years and never found it very useful (or fun) ... maybe when my second DS was in K and working on Mathletics set at Year 3 it was sort of ok but that didn't last for long once he had another big leap forward. Also, do you find it teaches him anything? I find it to be practice not a teaching new concepts tool but perhaps I have totally missed something!
I think you would find it interesting to give him the Singapore Maths placement tests (
http://www.singaporemath.com/Placement_Test_s/86.htm). They are free, you can do it at home just as a data point for yourself.
