@Gnarly999
What an insensitive arrogant piece of shit, to send you a message like that in the middle of the night! He was hoping you'd sleep right through and not see it til morning when he could avoid your calls.
So, now he's said what he said. And I agree with PPs, he was being a prick in the hopes that you'd end the marriage. You didn't so in order to end the marriage he's had to 'come out in the open' and say he was done. Can you accept his words 'unconditionally'? By that I mean accepting that there is absolutely no way back, for him or for you.
I also agree with PPs, please don't confide in your MiL. At the end of the day, he is her son and she will take his side. Not that she'll put the 'blame' on you, just that she will find justification in her own mind in order to 'support him'. And you cannot trust that things you have said will not get back to him through her. In fact, I guarantee that they will, if only because she's told him to try to get him to see your side or to double check that things you've said are accurate. So be polite, but scale back your conversations.
So now, take a deep breath and start making serious plans. And to understand that you are not 'giving up on your marriage'. He is done, and you can't save a marriage by yourself. It takes two willing partners. When he gets home, there need be no 'serious discussion' or 'having it out'. Behave with dignity. Step back, stop providing any and all domestic services. He wants to be single, fine, it starts today. He can cook and clean up for himself. He can do his own laundry and life admin. You take care of yourself and your DC, period. And go places and leave him with the children, even if it's to a friend's or relative's for a couple of hours. I know you can't leave the baby, but he can watch the others. Simply say "I'm going out, I'll be back in X hours". He doesn't need to know where you are. This is part of your 'internal learning' that where you go and what you do no longer has anything to do with you.
And now is the time to see a solicitor. No more burying your head in the sand. You need to understand what divorce will mean to you. And if he tries to discuss 'terms', agree to NOTHING and sign nothing until you've had legal advice. Yes, I know that seeking legal advice feels like crossing the Rubicon. But he already has. Time for you to catch up.