I'm sorry about your DS's troubles, cym.
Both Singapore and Aleks are good programs. There are pros and cons to each, depending upon your specific situation.
Does he (and do you) prefer workbooks or computer work? That's a key (and simple) distinguishing feature between the two, I think.
Aleks has a free trial, so that's handy if you want to see what you think of it before you pay anything. It does have a monthly fee, whereas for Singapore you only pay for the books you use, so for afterschooling, I would guess Singapore would probably be cheaper. (For full-on HSing, I think the cost issue would be less of a factor, since we go through the books pretty fast.)
Singapore tends to have a different slant on math than most programs I've seen. More "mental math." Fewer practice problems and less review. That can take some getting used to, but it is probably also good if you're trying to help a kid who is frustrated by the usual approach.
As for which Singapore workbook to choose:
* "Intensive Practice" and "Challenging Word Problems" are good if you want to challenge a child with more critical thinking work and less arithmetic--and sometimes challenge them a lot, because the problems can be downright hard. The arithmetic isn't hard, but you have to do some serious lateral thinking to figure out the solution!
* So if you're looking to push him in the area of problem-solving, then I'd recommend "Intensive Practice."
* If word problems are a good way to reach him, then "Challenging Word Problems" would be my first choice.
* If you just want to make sure he's got the basic math skills, then the regular workbooks are probably better.
If you decide you want Singapore, be sure to check their website so that you get the level of workbook that's right for your DS. 4A is really more like 2nd semester of 4th grade rather than 1st semester. The website tells you what's covered in each book so you don't buy the wrong thing and waste your money.
I know I'm not strongly recommending one thing over another, but that's because your needs and wants are what will distinguish between the curricula choices.
I hope I haven't totally confused you...
