Khan Academy has a knowledge map of math subjects that lights them up one by one as you show mastery of the earlier topics. Each topic ties to the correct video lecture(s). You can still bypass and go directly to a later subject.
Go to the Practice tab or http://www.khanacademy.org/exercisedashboard?k
Your child will need a login, meaning a Facebook/Google account (the legality of which, especially for children under 13, still being iffy).

Nice interface, love the integration with the lectures, but (I have been playing with it rather extensively from the bottom up) and proving mastery is rather tedious. Frankly having to do 10+ long divisions by hand is mind numbing (and one careless mistake sets you back a lot).

I find proving mastery in Aleks (from watching my son play) much better -- you only have to get 4 items correct on the first try, 2-3 more if you make a mistake, and there will be one review with mastery proven in one correct attempt. The system also remembers when you "nearly" got there and will qualify you on way less than 3-4 attempts when you try it again. This has proven crucial for my son, whose performance tends to spiral down fast when he starts making careless mistakes.

You can get a 2 months free trial of Aleks here: http://www.aleks.com/webform/c215
takes a few days to get activated, and you will loose anything your child did on the first trial (I couldn't figure how to do the longer trial without creating a new account), but hey, two months!

Aleks recommends that you start at your current grade level and then move up (of down) if your initial assessed results fall outside of the 25-85% range. If he rated a 65% your son will be fine at 5th grade (will probably finish it quickly, and then you can move him up).

Here is the reason why (because yes, I am that kind of anal, Excel spreadsheet available for those who share my issues ;)):

3rd grade math has 127 topics.
4th grade math has 196 topics. 125 from 3rd grade, 71 new.
5th grade math has 266 topics. 196(!) from 4th grade, 70 new.
6th grade math has 324 topics. 266(!) from 5th grade, 58 new.
(I am still trying to figure out why MS1 is 95% repeated material rather than "only" 82% for 6th grade)
Lots of review/repetition built in, Aleks was designed for students operating at both ends of the curve - several grade levels above/below the norm.

When you move from one course to the other your child's mastered topics are supposed to move with him (per support -- we are not there yet). So after mastering 5th grade you should start 6th grade with a pie about 80% full.

Now if only Aleks could provide comprehensive (the existing is rather skimpy and too much based on the current problem), multi format (text/video) explanations it would be perfect.