When I was a kid I’d resist bed time because I didn’t really feel safe that far away from my family. Once my parents went to bed that end of the house was safe, but when they were still up my bedroom felt very isolated. And the same thing happened in the morning - my sister and I would usually bring our clothes to the lounge room to get changed rather than be alone in our rooms.

If it is something like that he might be able to sleep closer to you and then move (or be moved) when you all go to bed? Or maybe if you have baby monitor you could set that back up so he feels more secure?