I really like www.aleks.com, which is mentioned in the FAQ by Mr. Bunday. It is $20 per month if you do it privately, but if your school/teacher orders it, it is only $35 or so per year !
You can log on for a 48 hour free preview trial.
Aleks is a regular tutorial site.
See if your son could do it for like 10/15 min each day at school.


Also, check out www.beestar.org
Their spring session starts this Sunday. It covers not only math, but grade level math is free.
Those are only excercises with immediate feedback.
He could also do them at school.

Again, Singapore Math Series, especially challenge problems books. Forget about any other workbooks. If you buy any, buy those.

Good luck LMom :-). Please remember, that the most important part of a good math education is the understanding of "why". Do not rush forward, go deep and wide :-)(but truthfully, if you know how to add, you know how to add and then you have to go forward ) grin