I dont think its a bad thing if he's nagging his mum to have the kids. You've been considerate to her and that's a good thing. I think you said she understands that you need to work and its better for everyone if you do.
I think she's going to give him a talking to about this soon and she's in a better position to do so and he will quickly come unstuck if he starts giving her the same shit he has with you.
If his behaviour is inconsiderate and annoying to his mum. Let him.
She knows you are more accomodating. I think she can see you are all about the kids, he is all about his "free time" .
She's helping him out atm because of his sob story what ever that is, but its getting old now, especially as he has the day off work that day... so in her view why is he dumping? His actions are now beginning to speak louder than his words, even with his mum.
Obviously you have to do what's right for the children, but I don't think you have to go out of your way to protect him from annoying his mum. If his behaviour is making both you and his mum go the extra mile, he should feel the consequences of that (provided the DC are ok of course) because he doesn';t have to operate like this and it might actually wake him up. He chooses to. Because he's selfish.
She was shocked he hadn't told her about the divorce. perhaps because he said this is just whilst we sort things out (translation.. this is just whilst I establish 50;50 to avoid paying maintenance and after that I can do what I like) and perhaps she's now realising that this is more permanent than she was led to believe? Just my speculation of course.
I do agree with the other pps who have said we've never met him but he's bloody annoying.