In reply to fauves:
Once he is over 18 anywhere that employs him would be expecting them to be working towards or have national governing body qualifications.. ML etc. Diversity is key, don't just pursue one activity. Apart from the obvious outdoor sports, many centres offer caving, archery, sailing etc.. Google the kind of places you or he thinks he might enjoy working at in say 3 years time and see what activities they offer. Or search for old job adverts.
Be wary of the kind of courses that charge a couple of grand, with the promise of some low level quals.. in many cases you might be paying for the privilege of working for them everyday with their groups. Just ask all the obvious questions.
It's tough for the young to go straight into outdoor work without being used as a slave for a few years. It's a lifestyle choice though, they'll barely make enough to keep him in beer, but with the right centre they will develop his group, personal skills and qualifications, it's purely down to how good or keen the senior staff are.
Aspirational stuff have a search for the so called night watch / junior staff schemes at glenmore lodge or plas-y-brenin. Not many places and lots chasing though.