Actually, I wasn't thinking about her pulling out of helping, but more about his mum seeing first hand how self focused he is, because that will be impacting on her and she may either have a right go at him (and at the moment she does have that hold over him because he needs her help) or start insisting he pulls himself together,
But overall I suspect he's just winging it as he has no clue the level of thought and organisation that is involved.
But your little ones are so lucky they have you as their reliable permanent support. You are doing a great job, and it will get easier with time, as the wrinkles get sorted out.