My advice as he is still only 15 would to be start the PPL course and continue schooling, he might not enjoy flying and at least you can find out yes or no whilst still at school. If he enjoys it, he will have the first part of course out the way and can continue building hours and experience whilst still in education, he can then look at what options are open to him further down the line, sponsorships or mentored schemes may be available again at that time.
I see your in Hampshire, there are a number of schools at Bournemouth Airport that might be worth a visit.
Bournemouth Commercial Flight Training. Commercial flight training in the UK. CPL, ATPL, Ground School, Flying Instructor and Examiner courses.
EPTA
Professional Air Training: Professional flight training school