I see this quite often in students on the spectrum. Two key things you've mentioned:
1. categorizing quite unconventionally, often based not on what we might call in NT-land the essence of the thing. There's a famous (possibly apocryphal) story ascribed to a number of prominent HFAs, about the concept of "cat". Instead of an idealized (Platonian, one might say) image of a cat, it's more like a slideshow/file of many, many specific cats, which have been individually tagged as "cat". In the absence of a specific tag, there isn't an automatic conceptual quality that stands out. So any given tag could be the one reached for.
And yes, sometimes it's just divergent thinking. But I think you're seeing a way of categorizing that is based on bits and pieces that NTs would view as non-salient details, rather than conceptual categories, which is much more ASD in nature.
2. organization in written expression. Executive function is a huge part of writing, especially once one gets past simple sentences, and especially for persuasive writing.
What you want to go to the district with is a request that they remediate the impact of the executive functions known to be associated with ASD in the context of his academics, especially in written expression. EF includes many skills necessary for written output beyond simple sentences: idea generation, initiation, planning, organization, to name a few. He also may struggle with cognitive flexibility once he gets stuck somewhere in the writing process.
They'll be familiar with the impact of EF on work completion, but my experience is that it also hits pretty hard in written expression. But it can be taught. One of my recent evaluations was a lovely ASD student whose essay writing has soared from well below average to totally on grade level over the course of high school. Direct instruction and consistently scaffolding EF with graphic organizers, rubrics/checklists, and cues until it becomes second nature will work wonders.
