The KTEA-3 writing fluency subtest is handwritten, and literally is a measure of how many words of meaningful language the student can generate by hand in a given time. If he is doing well with the provided accommodations, especially the kind you've listed, that is good documentation that he requires specialized accommodations to access his education, which meets criteria (with his diagnosed disabilities) for a 504 accommodation plan. If he doesn't need skills remediation (specialized instruction), then he doesn't need an IEP. In younger students, OT interventions make more sense, as there is still some possibility that they will gain functionality. By the time you hit middle school and beyond, it should have shifted over to primarily instruction in using relevant assistive technology. Once he's fluent with his AT, he may very well reach the point where accommodations are sufficient.
If you have lingering concerns about stealth dyslexia, the appropriate measures would be of reading fluency and of nonword decoding skills. The KTEA-3 has additional measures of silent and oral reading fluency, of word-level reading of real and nonsense words, and of reading fluency with both real and nonsense words. The more important question is what useful action would result from a diagnosis of stealth dyslexia. It appears he already has access to audiobooks (and practically speaking, almost every suburban secondary student in North America has access to audiotexts--Pearson publishes every etext with standard audio), and doesn't need read-aloud when it's a shorter reading selection. Remediation would force him to re-learn more efficient decoding strategies, which would probably be met with conscious or subconscious resistance, since his possibly-less-efficient methods do work.
You could throw in some rating scales for ADHD. Those are quick, and give a bit more data across settings. The slow cognitive tempo facet isn't too unusual in ADHD.
