Just agreeing with others that if he isn’t longing for his kids, anticipating what they need to be at his, and missing them, wanting to be in contact, 50/50 must be a desire entirely motivated by Twattoo not wanting to pay anything. His mum will be doing the legwork and as your gut feeling is that she will run out of steam on this OP, I would say nothing for now, but assume that you will at some point in the future, be down to EOW. Possible that he could get a girlfriend who would pick up the slack for him in place of his mum, but it’s a lot to ask with 4 kids, so maybe not.
A pattern will emerge in time, keep a record of all the gaps, failed pickups, the refusals, and the early drops offs by him and when you know where you are, and the time is right - possibly after divorce/financial order when he has less to hold over you, file directly with CMS with your diary of what has actually been happening, documented. For now, (FWIW) I agree with being factual with the kids and encouraging them to ask him direct rather than you trying to cover for him and make him seem better than he is. The kids will work him out in the end, but encouraging them to ask questions and express their needs to him at the start is good for them I think as he will not anticipate what their emotional needs are at all. It’s heartbreaking.