Op, I have followed this thread from the start and I do think that the missing key in this is his brother.
Until this September they got up together, hung around at home together and went to school together. After school they were then together.
Your older son was acting informally as your younger son's babysitter/childminder.
Since your older son has gone to sixth form, your younger ds is struggling to cope. You really need to sit up and take notice of this.
My ds is 12.5. He is pretty together and organised, but he would really struggle if I stepped out of the picture for so many hours in the day. He would not get himself out of the door if no-one was watching. He would not put his phone down and get his homework done, get himself to bed and lights out if there was no adult there day after day. He is too young. More than that he needs the support and the company.
I send him to bed at bedtime (decided by me) and insist that lights are out. he has no electronics in his room after bedtime, as he would just turn them on and play/watch them. For this reason he has no TV in his room, no tablet or phone and no x box. Why? because he would not self regulate time on them.
I think you need to tell him. These are the new house rules, bedtime at 9:30, lights out at 10 (and that is later than most of ds friends) No tv/xbox etc etc after bedtime. TV wires remotes etc removed until the situation has changed, eg give him 3 months to prove the new regime works and then allow some things back. School everyday, each day late/missed means the electronics restrictions are extended by a week.
And at the same time, he needs more time and support from you. He is struggling because he has too much unsupervised time. If that means you need to look at your work hours, then you need to look at your work situation.