Perhaps specifically for the writing section? Doesn’t seem to me that the other sections emphasize English grammar. The predecessor of the writing section, an earlier form of the Achievement Test in English language (not literature), was a multiple choice grammar test, so perhaps the grading of the successor essay-based tests emphasizes grammar quite a bit.
So what was his SAT score after preparation, and for how many weeks was this preparation? If he was already at the 2100+ level without preparation, the relatively limited additional gains possible from preparation make the value proposition of spending many weekends taking practice tests to be less than appealing.
Seems quite expensive. Of course, your sentence implies that he got in without you needing to spend $3,000-$5,000 on test-prep.