Haven't run into the situation yet, but I have my parental response queued up as: "Asking my son to 'show his work' when he has no work to show is asking him to lie. That goes against our values. Can you please not teach dishonesty?"
Other argumentation points...
"Number sense is an important skill/talent indicator for future mathematical ability. Automatically knowing answers and fast calculating in one's head all build number sense skills. Slowing down to force non-existent partial steps in to a solution undermine the healthy development of number sense."
"Learning math at one's proximal zone of development would be the best way to generate enough challenge that intermediate steps would materialize. Rather than him 'showing his work' how about you 'doing your work.'"
Curse you currently awesome school that has undermined the need for all my snarky comments!
I've also told DS that breaking problems into smaller steps is a way to explain problems to people who don't understand math the way he does.
