Although one major thing also is that my wife had to go away to work overnight, she's been WFH for months but she's on a big job where she has to travel and stay away Monday nights now. Not ideal at all sad this started after she went away last week and tomorrow she's off again.
There you go, I’d bet my house on this being part of it - remember he’s used to people disappearing on him so may well be panicking or fearful that she won’t come back (on a very basic level). I had to be away from home around the same point in placement although my two were older.
He might be too young to understand the idea of a transitional object but could your wife give him something to keep safe while she’s away (so that he knows she’s coming back). My kids would give me a favourite cuddle toy and I’d send photos of them doing stuff like sitting on the train with me, next to me in “meetings” etc and send them back. We also developed a special way of saying goodnight that we always do - so if I have to be away I’ll always phone at bedtime and say our special goodnight.
We also have a calendar marked with the days I’m away so they can literally cross them off. When I’m getting ready to go to work I’ll talk about what I’ll be doing, and what they’ll be doing and how much I’ll miss them. When I come home we talk about what I did, what they did and how much I missed them. The idea is to build bonds that hold while she is away - literally the definition of attachment. When he wakes up it might help to chat about his mum, really light tough but “I’m really missing mummy, let’s look at her picture, and see when she’s going to be home again”. The nature of insecure attachment is that children think if you’re not there you don’t exist anymore and developmentally he doesn’t quite have capacity to understand that she will be different to other carers who have gone.
He needs to know that she comes back, and he doesn’t know that yet so bedtime will be tricky for a while, until he knows reliably that she comes back. It comes up at bedtime, I think, because that’s when they are tired, and vulnerable and when they need their carer.