Your response was definitely reasonable and polite so there shouldn't be any blow-back on your DS.
His teacher should have written "show your work" on the instructions and gave the students actual examples in class rather than just a verbal instruction. I am not debating the appropriateness of showing work in this case, but after having had three children complete elementary math curriculum, I can see immediately what your teacher likely expected:
20-12 = 8*3 would be "False because 8 =/= 24"
7 = 14*2 would be "False because 7 =/= 28"
497 < 500 would be "True because 497 = 500 - 3"
