My 9 month old keeps waking up 4/5am

It could simply be he has had enough sleep and his body clock is waking him up. Maybe try putting his bedtime back by 10-15 mins and see if that helps, and if it does move another 10-15 mins later until you get the wake up time you want.

What are naps like in the day?

Make sure your child naps right in the middle of their awake period in the day. You neither want your child to nap too soon after waking so that they then have a long awake period before bedtime, or too long after waking so that they are cranky going into the nap, and also do not have long enough time awake before bedtime.
Take a look at his total sleep time in 24h, sometimes it’s unrealistic to expect a child to sleep 11-12 hours overnight.
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I am inclined to say it’s hunger, and will require more than milk, I would suggest offering warm weetabix before bed.