What @PumpkinPie said is very good: Don't worry about how to start the conversation - just say 'I've decided to leave X ' the rest will follow from there. If you want a lead in to that, you can start by saying, "Listen, the DC and I need you for a couple days..."
If you think he will be monstrous about it when you tell him, then maybe text her now, before you get home. Perhaps arrange it so that she's there when you tell him. Maybe she can keep him from verbally abusing you, and if not, then she will see just how bad he can be.
Then, if you need to, or if you get annoyed with her while you're at hers, or if she for some reason decides to take too much pity on him, then you can all go to your mums. It's not far, really. And you've got a good lead and time to make preparations from his mum's if you need to.
Good luck, and here is my handhold, @bodytrapped. You've got this. It sounds like you've got a good, solid plan and IRL support to boot. That's a damn sight more than a lot of us ever had in your position, and we made it through and here we are. His mum knows what he's like, and she will listen to you, even if it were only for the sake of her DGC. You've got this.
You'll be okay - and better, in fact - very soon, and your DC will be okay.
Good luck, love.
We'll be right here if you need us.