I thought the plan was Dad would pay uni fees - but, especially if DS will be living away from home like he wants to, there will be many other university expenses? Accommodation to start with…
OP said they are paying his fees, bought him a car and pay some of the running costs and provide him with an allowance (which will help with living expenses). He also works. Presumably his mother should also contribute towards his university costs so perhaps she can fund the accommodation given it appears the rest is already covered by the DSS himself/ his father and step mother? And they are apparently simultaneously going to save £50k for a house deposit for him.
Surely it should be a balance between the two parents? Why is their contribution insufficient? The mother should be supporting him as much as she can herself and will have no expenses any longer in relation to him except visits in holidays.
Apparently he splits his time equally between his parents' homes so presumably he'll continue to do that during university holidays, so it's hard to see a case for the father to continue to pay money to the mother's to fund the DSS's living expenses in this situation as she's no longer housing him/ paying for anything to support him that the father and step mother need to contribute towards.
I am disappointed to see some of the attitudes on this thread. I think the amount of CMS required by law is an insulting pittance and not enforced properly but from OP's posts she and her husband have gone far above this and intend to continue to do so, helping her DSS very generously. Why the mother feels entitled to continue to receive money from them in the above circumstances is a mystery to me so please can somebody arguing for this explain the reasoning?
As a lone parent raising my children with no involvement from the other parent I think this type of attitude - that somehow this mother should continue to receive money when she has no additional expenses to fund for DSS - quite shocking and it undermines all of the totally valid arguments about deadbeat fathers who do not pay anywhere near what they should, or nothing.
Maintenance is to fund the expenses of raising the child, not to fund the other parent. It should be much higher than the CMS amounts, and equate to 50% of the cost of raising and housing a child. But this child has been raised (with very reasonable contributions to the expenses involved, it seems) and is now an adult so there's no further case for maintenance, surely?