hi KB
I strongly recomend your approach re the gliding.
I have a fair bit of experiance working with 15-19 year olds in both powered flying and gliding.
One of the main issues with adolesants in INTREST, which is likely to change over time, even for the most interested young person.
The fist recognisable step in flying training is the SOLO, this is achived at an earlier age in gliding and therefore keeps the intrest up, the following stages of recognition are also not far away inters of flights, time and money. For young people this recognition and intrest keeping (focus) is important.
Many of the skills learnt in Gliding (Hand -eye) and visual awarness of height and distance are 100% interchangable between powered and gliding. but the most important skills of judgment and disciplin in flying are undobtably higher in the glider pilot in early training, in fact much of the early training is completley about these thigs.
There is, as shown by an earlier poster, accreditation given in both the Regs and the training syllabus for a person who has recognised skills and demostrates them during training, going either way.
All the best for the young fella, and to echo someones comments earlier look at the cadet system for both knowledge and skills training.
richo