You know it can be very difficult for anyone, especially kids to suddenly change the sleeping time. So to avoid any problems with sleep time and the hours they get, I'd say you start a new routine at least two weeks before school starts.
Let him have an early dinner, by around 8ish, so that he is in bed and asleep by around 9. That way, even if he wakes up at 7am, he'll get 10 hours of sleep. Instead of making him sleep after he's back from school, you can use the time to finish studies and let him play. This routine works best for my kids too :)
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