If he's already completed his A-levels, and gone on to
higher education, your payments should already have stopped when his A-level studies did UNLESS the resident parent is registered for Child Benefit in respect of him (in which case your payments will stop when he reaches 20).
If he's currently studying for A-levels (or at an equivalent or lower level) then, assuming that he continues with those studies, your payments should cease when he reaches the age of 20.
https://www.gov.uk/child-maintenance/eligibility