An alternate perspective: my PG sib did not write down any math reasoning until calculus (well, maybe for geometry two-column proofs)--mainly because, why, there were no steps! Went on to collect multiple STEM bac and post-bac degrees. And actually, had no issues with demonstrating reasoning at the collegiate and higher level. When my sib was age 9, my parents were still advocating for grading on accuracy of answers only...and take your lumps (i.e., no partial credit) if the answer is wrong.
It is not necessarily the case that your DS does not know how to write down steps. I'd want to probe a bit, first, to see what his actual process is, or if it just seems too obvious to him to spell out.
