There’s a lot of unknowns here so hard to offer advice, but I would not be paying CM anymore at all, however….
Mum needs to communicate what she expects from DC. Does he need to give her money toward his food/accomodation or is she willing to support him with that whilst he completes his apprenticeship? Maybe she agrees to that, but you then cover his other bills?
As the paying parent, I would be helping my DC financially through the apprenticeship if I was able, but again, however…..
He must have some form of contract? Or who is providing this course? You can look up what it entails yourself if there’s no documentation to view.
Is the apprenticeship full or part time? If part time, they need to get a part time job too. I would look to top up what he needs after that.
You need to look at his necessary expenses (phone/car/clothes/food etc) and offer to help cover some/all of that depending on what he needs so he can remain on the course, but not so he ends up not valuing the support or just expects it with no gratitude.
If mum wants starts demanding from DC what you’re no longer paying, can DC come and live with you?
DC is an adult now, so it’s down to you/his mum how you each support him completing this apprenticeship, but you shouldn’t be paying his mum directly anymore to do that. DC needs to start learning about income/expenditure and how to manage that himself.