Whenever I find an example “in the wild” of writing that needs to be improved, I save it to show my 8th grader. On Facebook, you can find infinite examples of people mixing up their, there, and they’re. But I also come across writing that’s awkward or just plain wrong in my local newspaper and the online edition of national magazines. Often, the problem is subject-verb agreement, when the writer has tried to put so much information into one sentence that the subject and verb are too far apart for the writer to remember what he was talking about. (Feel free to have your student try to improve that last sentence!)
You can also look for examples of student writing and have your ds try to improve them. He might learn to recognize his own shortcomings by seeing them in other students’ writing. Try showing him lower-scoring examples from the AP Test website, or search for examples of college application essays.
Michael Clay Thompson has a book of student essays meant to help parents figure out how to grade essays. Reading the essays and Thompson’s comments, and trying to fix the essays, might help your ds.
https://www.rfwp.com/series/writing-program-by-michael-clay-thompson#book-opus-40-a-resource-for-grading-academic-writing
Hope you get some more ideas! It would be great to find a source with tons of helpful examples of writing samples that need improving.