He doesn't have to pay at all through CSA, but I think the child can make a claim in the courts for support until s/he leaves full time education. It would probably be more sensible to pay voluntarily. Most of us give our children some support through higher education as they just can't manage otherwise.
In England & Wales there is no legal obligation to maintain over 18s unless there is a court order in place. When there is a court order the terms of the order will be relevant, although as Sadie says if there is no existing order a 'child' over 18 may be able to apply through the courts for financial support in their own right.