I have insomnia (or delayed sleep - unfortunately, the local NHS services see any middleaged woman with sleep issues as needing antidepressants, so won't refer me for further investigation).
If it were left to me, I would go to sleep late and wake up naturally. But I can't. I have a job.
The way I manage it is by NOT SITTING DOWN when I'm tired. If I sit down, I doze off - and then I'm bolloxed until about 5.45am, whether it's for ten minutes or three hours. Obviously, going to sleep at 5.45am is a really bad idea for maintaining employment.
The other thing I do is work in multiples of 90 minutes. There's something about human sleep cycles lasting that long - if you are woken up before the lighter sleep that happens towards the end of each cycle, you feel like shit.
He wakes up happily at 6am. If you count back in 90 minute blocks, that means he's in lighter sleep at 4.30am, 3.00am, 1.30am, midnight, 10.30pm, 9pm and 7.30pm.
He has to stay up until 9pm. Which means being UP, not sitting down. Have white/cool LED bulbs on, keep the house lights on all the time, including the Big Light if you have one. Obviously, once he goes to bed at 9pm, he'll be out like a light, but he'll still be able to wake at 6am because it fits in with his natural sleep cycles.
He won't like it. But if I manage to haul my carcass to work after 90 mins sleep - or none at all - and still deal with several hundred teenagers without killing them or going insane, he can bloody well make himself stay up for 90 minutes.