I'm sorry.
I remember when my ex left - over a decade in - it was exactly as you describe. Out of the blue and while he was hanging around after announcing it, it was as if a switch had flicked in him and nothing about me and my reactions mattered. He acted as if we had never loved each other.
Within 24 hours I went from wanting to get away to wanting to reassert myself in that space - and actually that weird shift in him made it easier, in the long run, to stop minding.
There was another woman - he didn't tell me straight away but made no sense until then. Like she had just slotted into my place in his head.
If you can, focus on business.
Don't force him out tonight instead of tomorrow for the sake of it. He's still named on the tenancy. He's responsible for his share of rent until you update the agreement. He needs to clear his goods away. Leave him to it - arrange to visit friends if you can. Don't help.
He's obviously got somewhere to go / someone to go to. So he needs to understand that he'll be liable for child maintenance, since an infant won't handle 50-50 parenting. He's also responsible for communicating changes to your landlord. This is his mess - let him handle the pressure and not just walk away.
You may well feel a lot better without him and I hope you've good friends who will help you through.