There are actually good reasons for knowing multiple ways to skin a cat, as it were. At the basic math levels EDM may be focused on developing more number sense, ability to do mental math to check an answer, etc. You don't want a math student who can reliably calculate things a single way, but doesn't understand the reason for the calculation. Also, if they use EDM at your school, the regular teacher would probably be teaching the same material at this point. I'm not the biggest fan of EDM as used in our district, but your son's problems may be related to pace (especially if other kids have more fluency due to more practice), missing conceptual foundation (possible but unlikely I'd guess), or poor communication or rapport with the sub, who may be a bit of an unbeliever. The problem isn't really with learning multiple algorithms. Do you think it's possible he's refusing to do the work, or do you think he's doing his best?
ETA (on my computer instead of my phone now): I agree with Austin, as usual.
Singapore Math would be a great choice for supplementation in my opinion, since it tends toward clear answers and problem-solving practice, you can get extra Challenging Word Problems or Intensive Practice books, it helps a child build a strong conceptual framework (often by explaining things in multiple ways), and-- this is almost the best part-- I've found that it lends itself
very well to acceleration. SM has extra workbooks that you can skip completely, just going through the text, and you can if you like use the review and practice sections as pretests to see how well your son's mastered the material in a block. ALEKS is very good too, and there are plenty of other options for enrichment or supplementation. I also agree that math should be challenging, and while your son is very young you need to start helping him deal with his perfectionism now (here's a
recent thread with a list of strategies).
