So sorry to hear that you and he have been in such pain for these past few years. My heart goes out to you.

If you were to find a solid block of uninterrupted relaxed, quiet, low-pressure time, like during a long walk or over a snack at a favorite restaurant, and say, "help me understand what's going on with schooling--I'm just going to listen, and maybe ask the occasional question so I understand better," what do you think he would say?

I remember that you've reported that he has slow processing speed, and can struggle to organize his thoughts when expressing language on paper. It may be that he needs much more time to formulate his thoughts; the more challenging and complex the thoughts and feelings are, the more time he will need to pull them together in his mind, and communicate them. Good to hear he is in therapy. Does he have a good connection with his therapist?

And I would agree with Portia that an online college class might be a productive experiment. (FWIW, there are members of my FOO who were academically ready for college at the age he was when you started this thread, but didn't actually enter until early teens, precisely because of asynchronies in EF and social-emotional maturity.)

You may also wish to consider that the instructional level he has been receiving for the past couple of years is simultaneously well below his ZPD, and appropriate to his executive functions--while the intent may have been to allow him to fill in his weaknesses, he may be responding most strongly to the lack of challenge, which is impairing his ability to access his skills in his areas of weakness (e.g., emotional/behavioral self-regulation, and EF/organizational skills). Like many here, there is this ongoing conundrum: how to prioritize his mixture of strengths and weaknesses at any given moment.
...pronounced like the long vowel and first letter of the alphabet...