A lot of critical posts, which are understandable since teaching is one of the professions full of truly wonderful people that are completely let down in terms of pay and support in the UK.
I can understand that you want your son to use the gifts he has, to earn a financially comfortable lifestyle for himself, and in the UK, teachers are massively underpaid and under appreciated. If he wants to be a PE teacher in the UK, he would be struggling in a way he wouldn’t have to, if he’d chosen something else (and he could have been a lawyer/ analyst/ accountant/ hedge funder etc, etc, and probably been very comfortable, given his intelligence).
However, if this is his dream, you can’t really direct him to change it just because it (quite rightly) worries you. Instead, if teaching is really the dream, how about suggesting that he gets qualified and then works abroad in an international school where the students are taught in English (speaking here from Hong Kong) - the teaching environment is generally excellent, the students want to learn, and the pay is much better. He could choose to travel the world, there are so many international schools he could try for, which would be a huge opportunity for travel, possibly better than other professions could give him.
If teaching in itself is not the main motivation, can he look into roles such as sports nutritionist/osteopath/rehabilitation etc - support roles that might cater for his extra interest but with scientific knowledge, where he could maybe benefit from additional qualifications?
As long as he is well aware of the choices he is making in financial terms - although, to be fair, at that age, would any of us really have understood salary and pension differences? - you kind of have to go along with it and be his biggest supporter, but make him aware of the choices he has. At that age he won’t have noticed (for example) the highly qualified cycling coaches on the Tour de France, so can only think “sporty = school teacher” when there is a lot out there (and very financially rewarding careers) in terms of other supporting roles for sports.