igbu, the AoPS pre-algebra book is pretty comprehensive, so your DS could work with it without worrying too much about missing any pre-requisite knowledge. It's a great book to work through at your own pace, and allow your DS to wallow in the depth and the extensive challenge problems available.
If you are thinking of an on-line class, though, they move pretty fast and might not suitable for a child being exposed to a lot of the concepts for the first time. He might get the basic operations but miss the chance to really work at how to use them to solve the more complex problems (which is really the whole point of AoPS).
