Their maintenance loan for university is based on the household income of where they live, in this case, the other parent. The shortfall is considered to be the responsibility of the household of where they live. That would also include the income of any step parent living there. It might not be fair but that is how it is worked out. I personally think it should be based on both parents, not household.
However, my son gets minimum maintenance loan which doesn't even touch his accommodation, he is between £2k and £3k short and then he needs food, to survive.
So no matter how you feel about being cut out you should at least be prepared to pay some money toward his university costs. Maybe not the full CMS amount as that was based on them being the responsibility of their parent. The adult now has access to the maintenance loan and the opportunity to work whilst at uni and I believe around 60% of them do.
This may also be a case that they learn to live within their means and if they want more then they work for it themselves whilst at uni, over summer and breaks. Mine had 4 weeks at Christmas break, 5 weeks at Easter then basically 3-4 months off over summer.
I can see why you are reluctant to support them, this is a case of putting one hand out for cash whilst giving you two fingers with the other hand. Why should we allow adult children to treat us like we mean nothing and then say here you go, have this money?