I don’t know the cause, but there is definitely a family history of this for me. My dad and his brother were both IMPOSSIBLE to wake up. This was in the 60’s, so there weren’t a lot of options for schooling, and they both ended up dropping out of high school. They ended up working jobs on night shifts (dad was a cop after getting his GED, Uncle worked maintenance for airlines). They both had fulfilling family lives, although with upside-down sleep schedules. Maybe there’s a medical solution or psychological one, but I know for me, trying to go to bed early just doesn’t work. I can get up but it takes 2 alarm clocks and lots of time for “snooze” built in.
Maybe what your son needs is to adapt the world to accommodate this “disability.” There’s online schooling now that allows kids to get a high school diploma. If being at school is important to him, then yes, he needs to get up. But if not, look at the long term. He can get a diploma and get into college, and no, he doesn’t have to get up for 8 am college classes. I have a BS in engineering and I only had 1 8 o’clock in 4 years. More and more colleges are going to online classes or evening classes. There will always be a need for people who can work at odd hours (cops, firefighters, doctors, nurses, air traffic controllers, electronic technicians, gas station attendants, etc etc). He’ll be ok.
You need to be able to get this stress out of your life. best of luck to you 🙂