If he took the SAT, he already had to write an essay.
He took the SAT reading and math only, as part of his gifted program. I mentioned the NUMATS standards just as a reference (the test length or difficulty wasn't what worried me, just the writing).
I am feeling not very bright right now, but I didn't realize that the current SAT has a required writing section (it's been a very long time since dh, I, or our much-older kids took it). NUMATS mentioned adding on the ACT writing section for a fee, so I mistakenly thought that the writing section was only available for the ACT through NUMATS.
Although arcane philosophical discussions sound right up ds's alley ;), I think that life experiences at 13 would limit one's ability to write well on that type of thing, as opposed to the more straightforward ACT subjects, so it was helpful to learn the difference. Also, since we are doing this mainly to pinpoint his readiness and strengths and weaknesses, I think it would be more helpful to have him scored separately on each section.
Thank you so much for the input!