Academically, 8th grade is a pretty flexible year. Personally, I wouldn't consider any academics at that grade an ABSOLUTE. People will sometimes take the year to do something really DIFFERENT, knowing high school is coming. Practically, functionally, it sounds like you might need a grade adjustment, like he might need a year to sort through these issues before he's ready for high school. It could be an option on the table. More bake time can be good.
Where are you at with the autism question or using autism strategies?
I really don't remember all your posts, though I see your name a lot. It looks like you've been down a lot of paths, so I'm just throwing out things, running cats up the flag pole, as they say.
-interoception--It sounds like he's struggling with this, so he's not self-monitoring, not realizing when he's tired, etc. The mindfulness component you do as part of working on interoception could bump the overall attention and ability to plan, etc. etc. as well.
-transitions--It sounds like he has difficulty with transitions. Joyce Snow has a really helpful explanation of this in her book Teaching Your Child with Skill and Love. She explains the neurology behind it and why some kids continue to need varying levels of prompts. I didn't understand it until I saw some kids at an autism charter needing prompts (literally a physical touch to the arm) to be able to pick up their arm and strike the key on the keyboard. That's an extreme example, but I realized that we have other levels of that in our house, that sometimes people need a prompt or need supports for transitions.
-whether the ADHD meds are stimulants and making the insomnia worse--I'm just asking, just throwing it out there. My ds is one of those who looks ADHD who actually tests with great attention and normal impulsivity. His body has a lot of movement, and he has a high need for movement. It's becoming harder to GET him that level of movement.
-retained reflexes--Again, you're so thorough, I'm assuming you already checked into this.
I think you might like to look at 360 Thinking, watch their webinar ($39), and see what you can carry over and apply with him. I think it would be a good time to work on your work strategies, your EF supports, how you work together. It would be better to spend 8th grade really working on that and really getting a paradigm that works for him. Then you know you can plug subjects/materials into that paradigm and have it work.
As far as his body, who has he seen to work on that insomnia? It sounds horrible. I assume they've done the basics like checking b vitamins, d vitamins... Interoception, becoming more aware, finding strategies that work for him to calm his body down into a restful state...
My ds finds it almost impossible to go to bed if he hasn't had enough physical activity. So like last night I took him to the Y and he ran laps. He sprinted his laps then would rest while I did core, then he'd sprint more laps ahead of me, hehe. He ran a mile that way, which isn't so much, but which seems like a long way and a lot of laps when you're 8! We also played basketball till he was good and sweaty. With that, he could go to sleep. But we had to do those laps in small amounts to keep him calm. His classes are usually long but have breaks while you wait in line, etc. That's why I had him completely resting while I did core after each set of laps.
You mentioned in another thread he's low tone. Does he shy away from physical activity or fatigue easily? Coconut milk can be good for tone. You could look into the MTHFR thing. My dd is like that, and she just started some methylated folate, thinks it's great stuff.
Our ped is telling us insomnia, or at least night owl, is normal to teens. Maybe, but if you've got the teen thing PLUS some interoception or inability for him to feel how he feels or what would make him feel better, then it's a mess! He's old enough to go do weights, use a weighted blanket, and make choices like that if he had some instruction and supports. I'm looking at getting a worker to work out with my ds, because I want to work on that. We need it to move from him doing it because people told him to to him doing it because he's making the choice to do things that make his body feel better.