How to help baby nap

LO was born premature so he’s 23 weeks, 16.5 weeks adjusted. He started sleeping through the night 3 weeks ago, 11-12 hours straight, no night feed and some mornings I have to go in and wake him up so his night sleep is great.

However, his naps took a sharp turn this past week. He will ONLY nap with me and on me. He will no longer let his dad even hold him for a contact nap. We both used to be able to put him down in our bed or his crib but now that is not the case. We never formally sleep trained so it seems like we’re going to have to as I can’t always be available for every single nap.

Forgive me if I format this wrong but nap schedule looks like this right now. 1.5/2/2.5/2.5

Is this normal at this age? Any sleep training education and/or guides you’ve used? Open to all advice! One thing I know I absolutely cannot do is CIO but we’d be open to something modified.