It seems the pitchfork mob has come over from AIBU. Such a shame. Wind your necks in please. OP is asking about what her brother's legal position is.
The answer, as some have correctly posted, is to be found in the Child Support (Meaning of Child and New Calculation Rules) (Consequential and Miscellaneous Amendment) Regulations 2012 (SI 2012/2785) which amends s55 of the 1991 Act.
It states:
(1) Schedule 1 (meaning of “child” for the purposes of the Act) to the Child Support (Maintenance Assessment Procedure) Regulations 1992 is amended as follows.
(1) A person satisfies such conditions as may be prescribed for the purposes of section 55(1)(of the Act if that person satisfies any of the conditions in sub-paragraphs (2) and (3).
(2) The person is receiving full-time education (which is not advanced education)—
(a) by attendance at a recognised educational establishment; or
(b) elsewhere, if the education is recognised by the Secretary of State.
(3) The person is a person in respect of whom child benefit is payable.”.
s55 states:
(1) In this Act, “child” means (subject to subsection (2)) a person who—
(a) has not attained the age of 16, or
(b) has not attained the age of 20 and satisfies such conditions as may be prescribed.
So, as has been explained earlier, if the course is full time he cannot be compelled to pay maintenance. The child's income is irrelevant. If he is in full-time scondary education then maintenance is payable until his 20th birthday.