The one remaining unknown (unless you have CELF results from the previous eval; I can't quite tell, as you only listed artic assessment, but that might not have been the whole of it) is whether there are genuine expressive language challenges too. I wouldn't think so, given his KTEA-3 OE score, but the CELF is a bit more in-depth, so it's possible there are subtle deficits or relative weaknesses that would show up there, but not on achievement testing. I would say that that is the one remaining area that one might consider looking at, if one were going to do any more testing. I'm not advocating strongly for this one, but fwiw.
Graphic organizers: also called mindmaps, idea maps, etc. Some are even more specific, and can be converted directly into a paragraph or multiparagraph product, either by hand copying or clicking. It's unclear how much of a factor organization and planning is in his writing (not much paragraph-level writing is in the KTEA), but certainly, if one has to devote a lot of mental energy to the physical and technical mechanics of writing, not much working memory is left for holding one's ideas and writing structure. Externalizing that into a graphic organizer might help. I'd consider both paper and electronic graphic organizers (he can try them and see which ones work better for him).
Clean copy of notes: he's a bit young, so notes probably don't come up much now, but as he goes into middle school, this will likely become a problem. With his slow, illegible handwriting, and rudimentary typing, having complete notes provided by the teacher after class, and skeleton notes during class, will be important. He has a really good memory, but everyone has limits.
I'm assuming he already has extended time, and you've already mentioned reduced work load. I might clarify that to:
Starred items only, sufficient to demonstrate mastery, no more than 50% of workload. Meaning that (like the existing 504) he doesn't have to do more than 50%, but he can stop when he's demonstrated mastery of the skill/concept. I prefer that the teacher star the specific items that will allow her/him to assess mastery of the learning standard. On a page of 25 items, one can usually narrow it down to five that will demonstrate mastery. The remaining are for practice and reinforcement. I typically tell students that they'll get a handful of starred items, which they should do first, and then if they have time, they should go back and do as many of the other ones as is reasonable, but not if it interferes with other important life functions (like eating, sleeping, having a life outside of homework). The extra items will give them more practice, and act as backup, in case they got one of the the starred items wrong.
Word prediction: I know he doesn't type well, or like typing, but adding word prediction might help, as he reads well, so he'll know if the word prediction is on target or not.
Spellcheck/thesaurus: So I know he has a deep vocabulary, and he spells adequately, but his spelling isn't keeping up with his vocabulary, which is why using the two in conjunction might help make his writing reflect his oral language a teeny bit better. I usually have students use the words they can spell (or get close enough that the spellcheck recognizes them!), and then use the thesaurus function to locate the more nuanced word in their existing vocabulary that they would have preferred to use, if they could have spelled it. Even though his spelling is not atrocious in isolation, it sounds like it kind of falls apart when he has to do anything else at the same time (which is typical for certain types of dysgraphics).
