Agree with people saying don't start too early, and/or don't expect actual bedgoing while it's light.
One thing that I did soon after he turned 6 , (5 ?), was explain that I needed "downtime", a "clocking off" moment, and so if he wanted a goodnight kiss and cuddle time, he would have to do it at a set time, usually around 20.30 after getting ready in all other ways, ( teeth, bed, hair brush, etc), and this usually works.
Then he reads or plays in his room, until his little alarm clock goes off at 21.30. Usually he crashes soon after. Simply removing my participation/presence/involvement helped to calm things down. He home-unschools so mornings are relaxed, but he is usually awake by 8.00 ish. So he is getting enough sleep.
Another thing that could be a factor is food. A bit of cheese, (if he isn't dairy intolerant), or other protein, for a little supper, can help quieten things down, rather than sugary or carby things.
Vitamin B's are important for good sleep; avoid sugar and white breads etc. Try to get him eating nuts or marmite/yeast extract, or beans/lentils, or wholegrain pasta/rice, or get vitamin supplements. It might make a difference.
At least half an hours direct daylight on the face each day is also supposed to help establish/maintain sleep patterns.
Good luck.
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