Albufeira may not be feasible if they're all under 18.
First of all, non-resident, non-EU citizens under 18s entering Portugal must be accompanied by a parent or someone with parental responsibility. In lieu of that, they can carry a signed letter from the parent or guardian, but it must name and provide contact details for an adult who will be responsible for them while they are in Portugal. And finding a place to stay, as PPs have said, can be a big challenge as even many hostels require an 18+ person present at check-in. They may find individual apartments, complexes, or even hotels that don't have a lower age limit or have 16+ but it'll really limit their options and there may be a hefty security deposit. And while clubs will generally let in anyone 16+ with ID, the legal drinking age in Portugal is 18 and it's usually (and increasingly) enforced in popular clubby places that attract international tourists (sometimes less so off the beaten path).
As far as whether he should on an international trip at all - if he seems sensible and mature, knows the people he's travelling with well (ideally you have met them too and don't have specific concerns about their behaviour or judgment), has a realistic idea (ideally, prior experience via family or youth trips) of international travel, and has the money, I'd consider it - just make sure he's prepared. And set guidelines and expectations about how often he'll communicate with you when he's away, and what he'll do in case of an emergency, as it's very natural to worry!