I support what Barbarian said. Reduce to 2 subjects now and continue later.
Not a teacher either. But I could have written much of your post, including the job interview bit. The standard advice we often hear about going for an apprenticeship and not being academic etc etc is missing the point about SEN. I would advise not being distracted by it. Changing a setting would be much more damaging than the alternatives. An apprenticeship involves getting and keeping a competitive job and combining it with full time studies. How is that more appropriate? If DS is in grammar school, he is clearly academic and belongs there.
I think not doing homework would compromise his grades, so eliminating it could be a pyrrhic victory, although it is possible. The key is to see what DS is struggling with time. Is he actually working all that time, why is he blocking? How long does he spend on devices? Sleep?
If he is really struggling with workload, I think you need to do something strategic and drastic now, as Barbarian suggests.
My DS, ASD, EHCP, grammar school, started to struggle in 6 form due to change in provisions, more unstructured self study and other factors including devices and romance. In short, his mental health deteriorated, he accumulated a backlog, lower grades than he was used to, low confidence, downwards spiral. We are still picking up the pieces.
The school anticipated he might struggle, so tried to reduce the number of subjects. In hindsight they were right. DS ended up doing A levels over 3 years, two subjects first, another later.
Whatever triggers it, the way children with ASD respond in 6 form in heavily related to SEN, so imo when they start to struggle it is best to limit the losses and consolidate what is solid for now, and deal with other subjects later. I know it sounds unpalatable, but it might be the best if not the only way forward. Alternatives might be illusory.