Downtime at night

My son takes a really long time to sleep despite having a set routine of him playing his toys, shower time, milk and getting into bed We can be in bed and all tucked in at 10pm and he’ll only fall asleep at like 12am earliest. If I start our bedtime at 9pm, he continues to stay awake until 12am or fastest 11.30pm Is there a way I can shorten this window?