6 month-old sleep

Our 6 month old wakes up 2-4 times nightly. He drinks enough milk and has started solids. We have a good routine but he doesn’t seem to be linking his sleep cycles (day or night) fully.

Context: He’s been in the hospital 3 times since birth (keeps getting sick at crèche) He’s a very happy and social baby We’re doing combo feeding (formula & breast milk)

What can we do?