He cannot really choose: either it’s specified accommodation within the HB regime, or other accommodation for which UC is available. If he is in specified accommodation, he cannot opt to have a UC housing element because of different eligibility rules.
If it is specified accommodation though, he IS eligible for HB. Here is the relevant bit of Reg 56(2):
(h)who is—
(i)aged under 21 and whose course of study is not a course of higher education;
(ii)aged 21 and attained that age during a course of study which is not a course of higher education;
This covers him until he’s 22.
