Thank you so much for getting back to me. But first, I would like to apologize. I am on vacation and when I first saw your question, i really thought it must be hearing related based on other posts. I should have acknowledged the trama you must be going through. To have a child of this age who is unresponsive has got to be devastating! I have a granddaughter of about the same age and to have her unresponsive to me would really hurt! So i really feel for you.
I think the audiologist will be important. Kids can react to noise and rhythm, and still not understand words. If he is basically deaf to the spoken word, that would explain a lot.
And yes, the autism spectrum is a possibility, and I can send you material on that. But reading you initial post above (several times), I also see things that would point to Sensory Perception Disorder. And that is something that a specially trained OT can work with (but they need to know that is what they are looking for). The problem with SPD is that the range of symptoms are huge. I am including links to two very good resources. Probably 90% of the symptoms won't fit, but you will recognize the ones that do. The links are:
https://childmind.org/article/sensory-processing-issues-explained/
https://www.sensory-processing-disorder.com/
Please let me know how the upcoming appointment goes. Best wishes!
Has his hearing been tested?