I agree, definitely worth the wait despite the time currently taken to reach the front line. Besides, if you want to do something exciting in aviation and have job security to 55+, meet lifelong friends and be part of a great organisation that's admired, a career as a RAF pilot is hard to beat - the airline industry doesn't offer all that, and certainly not now! And you'd need to spend £100k to train with the risk of discovering you've no aptitude.
Having an an ambition at 17 is uncommon. Encourage him to the full but do work with him to design an acceptable fall back plan and help manage his expectations - competition is fierce and I suspect will be even tougher until the airline industry recovers.