Not sure there is a lot to be done about it tbh.
It's as natural as, for example, sexual orientation, just the way some people are.
I still can't sleep before 2am, even when I need to get up at 5 (thankfully rarely!). And the same with food, rarely eat before 4-5pm, then dinner around 10-midnight.
Exercise helps a bit if I time it right - need to finish the exercise about 2-3 hours before I want to sleep, so I get a post workout boost, then drop.
Ditto hot shower about 30 mins before sleep, then your body is cooling at the time you want to sleep.
Yoga nidra/sleep meditations help a bit, but not sure how useful that would be for a 9yo.
But it's all pretty marginal, sorry!
Carbs can cause sleepiness in the daytime, but I don't experience that when I eat them at night, so that hasn't helped.
9 is old enough to understand and have the conversations linking sleeping late and feeling rough in the morning, so at least maybe you could get him intellectually on board with why sleeping late causes him problems for himself (i.e. you're not just trying to make him sleep because you want to stop him playing etc) - and see if he's willing to try some things himself to help with it, but if he's wired like that it's probably just going to be a bit painful living in a lark world. And with the hormonal and circadian rhythm cycles of a teen, it probably won't get better. Sorry.
On the plus side, you do learn to cope with the sleep deprivation, so as long as you let him catch up with sleep in the weekends, it's survivable.
That's a balance too though, cos left to my own devices (esp as a teen in holidays), I naturally sleep around 6am & wake around noon.... so best not to completely destroy the rhythms...
Hope you find something that works for you both!