Child maintenance

Hi could anyone please advise. My husband & his ex made there own agreement on child maintenance when they separated, he has paid this without fail for the last 5 years ( he is aware the CSA would have made him pay more) but she was happy with it & he has also always paid half towards school trips, passport, prom clothes, piano etc and anything else she has asked for as well as taking them abroad twice which he has solely paid for. His eldest left school 3 years ago & he continued paying the same amount. His youngest has just left full time education and is planning on Uni in September his ex has contacted him to say her maintenance money is to be reduced to the minimum due to household income (she remarried this year) & she wants more money from him. He was always happy to continue paying something to help his daughter out my questions are?
1. Does he legally have to continue paying although morally he wants to?
2 Does this money still have to go to his ex ( surely his daughter is now an adult & should be starting to take care of her own finances so pay the money straight to her.
3. Can is ex go to the CSA now to say he was not paying enough the last 5 years & make a back claim?
He has no issue at all still helping to finance her university studies but the amount he is already paying more than covers half the shortfall so is it down to him to fund it all as she will no longer be living at home? Is daughter has a part time job but has no intention of working any extra to try to get some savings now she's on 3 months holiday her words! He also gave her £2500 for her 18th which she has spent on concerts etc, rather than saving knowing she was planning Uni.
Sorry for the long post but he is worried sick now as we have just increased the mortgage to try & reduce the years & my health isn't the best & was hoping to reduce my working hours. Ps is ex already only works part time through choice.

Any advice would be greatly received. Thank you

In the circumstances you describe CMS would be payable until September if his daughter has just finished A' levels.

If his ex applies to CMS it goes from when she applies so no back dating.

What maintenance has ended for his ex?

Hi
Thank you for the response so technically she could only go now but it would end in September?
No maintenance has ended for his ex it's the student uni maintenance that is to be reduced because of her household income ,

Hi William
Thanks for the info. So even if she was to make a claim it would be from now until September at the most?
He still wants to contribute but she seems to be expecting him to up his payments rather decrease although as previously advised it already covers are 70% of the shortfall ,

Thank you William