Every night for 3 hours

If he’s having split nights of 3 hours every night then he needs less sleep than you’re currently trying for.

What’s his whole routine? Wake time, nap time and length and bedtime?

Aw bless him 😆 my 9 month old is definitely going through sleep regression ATM. He used to go to bed for the night at 6.30/7pm. Lately he's been having a nap at that time and then literally bouncing around the place for hours until like 10/11pm just will not sleep

Split night are usually a sign that there's too much overall sleep in a 24 hour period- they might need to drop a nap and the last nap of the day night be a bit close to bedtime (or bedtime may need to be bit later, depending on the current routine? Also my little boy takes his sleeping bag off so I put it on with the zip at the back so he can't get to it!