Yes, SI treatment can still help him. So can the Therapeutic Listening program. And interactive metronome, should he be able to participate, can be done anytime. It is used with adults all the time. I don't believe for a second that you've lost any window of opportunity.
If I were seeing him, I would still do the SIPT, even after 8y 11m. You can use it, you just have to qualify the results since he is older than the intended age. It sometimes is even more of an impact if an older child scores low when compared to a child of 8y 11m. The examiner just needs to note that in the evaluation, that the standardized assessment used is designed for younger children and the norms being used for comparison are for younger kids. The results can still be meaningful. And it sounds like Mite would still score lower, given your descriptions.
Did you check the provider list at
www.vitallinks.net for a therapeutic listening provider? Ask the OTs on your list if any of them do that. And while getting the SIPT done would really help clarify the extent and nature of the SID, he can still be accurately assessed without it by an OT with a lot of SI experience and understanding. So if you find one who seems to understand Mite right off the bat over the phone and is qualified to supervise Therapeutic Listening, but not certified for SIPT, then I would go for it with that OT. Get him started on the Theraeutic Listening.
The Therapeutic Listening activates that vestibular system through the auditory channels. They are one and the same system after all. And the time/space issues can really be addressed by the modulated music. I think you would be amazed at how the listening can impact a child's internal organization and sense of "groundedness" with the world. That alone can decrease anxiety in a child tremendously.
Sheila Frick and Vital Links are in Madison WI. She is wonderful! She developed TL and taught the advanced course I took on the listening program. Are you anywhere near enough to Madison to even see about eval at her clinic? I've sent them a quick email to see if they do evals/consults there. Have you looked at the website yet?