At the age of 14, your son is old enough to take the Powerboat Level 2 course.
He would do exactly the same course as anyone else, however at the end of it, his Level 2 Certificate would be endorsed "The holder of this certificate should only drive powered craft under the supervision of a responsible adult".
Note, this differs from the endorsement on the Powerboat Level 1 certificate for 8 - 12 year olds, which reads "The holder of this certificate should only drive powered craft under the supervision of a responsible adult on board the craft".
Not all schools offer youth courses, so worth giving your chosen school a ring to check.
Not so much of an issue at 14, but for younger kids (You can do a Level 1 at 8, a Level 2 at 12) it's also worth checking which boats the school uses, as some of the larger craft are a bit unwieldy.
Paul Glatzel (www.powerboat-training-uk.co.uk
) uses a 4m Searider for Kids Courses, Doug Innes uses a 4.5m Tornado (www.stormforce.biz)
, and at Haslar Sea School (www.haslarseaschool.co.uk
) we use either a 4m searider or a 4.8m Ribcraft. Those are the only schools I can remember off the top of my head that use smaller boats for kids courses.
The course is usually a good one, and covers all aspects of powerboating from slow speed maneuvres, to man overboards, high speed handling, and passage planning.
You could either send your son to a training centre for the course, or you could have an Advanced Instructor take the course on your own rib.
Hope that helps,