What next?

My little one is 5.5 months and goes down for naps and night time sleep independently. At night, we do our bedtime routine, put him down and then he’s asleep within 5 minutes with little to no fussing. Lately he’s been waking up anywhere from 30 minutes to 2 hours after he’s put down and then up every 2-3 hours after that. I’m stumped on what to do from here? I’m assuming he’s having trouble linking his sleep cycles and it’s a nurse to soothe association because he calms and falls asleep almost immediately after being fed. Is it time for CIO?