I remember particular 2nd and 3rd grade EDM homework that required the student to do the exact same two-digit addition and subtraction problems in literally SIX different ways each. Good to know different approaches, definitely. Necessary to do each approach with the same problem? I could definitely see where frustration would set in and even a complete disbelief that the work even needs to be done. As I explained to my DS at the time, however, sometimes you just need to jump through the hoops of school. And, unfortunately, a lot of school is hoop-jumping.
To me, though, it sounds like it could be more of an issue of both your son's perfectionism and his lack of challenge heretofor. I think almost all of us here have seen the frustration that accompanies the first challenge. Almost all of us here, though, have probably also seen that the frustration is short lived when they get slightly more used to the challenge and rise to meet the challenge and then, unfortunately, when the work becomes easy again (since they learn so darn quickly).
Since your DS seems happy when he talks about math at home and since you can see that he's up for the challenge, I think you should stay the course. Perhaps what is needed most is for you to attempt to educate the sub on how very common his reaction is for kids like him who are facing new and possibly challenging material for the first time.
