I'm sure it says in POR, someone who knows POR better than me will most probably quote it, but I think if you have approval of DC or GSL (it's one of them, think it's DC) then he can stay on for up to a year.
------ Update --- The Following was Added within 30 Minutes of the above ---
Flexibility for Individual Members
When necessary, due to a young person’s special needs, the District Commissioner may, on the recommendation of the Assistant County Commissioner (Special Needs) or County Adviser, permit a young person to be in a Section outside of the normal age range. This permission should be logged on a simple form and be subject to annual review.
A factsheet and supporting forms are available from the Scout Information Centre.