Honestly it sounds like there will be plenty of pluses for him being in Scotland, as he is sporty and interested in joining the military ultimately. Lots of opportunities for sports and outdoors activities, more so than Oxford.
I'd tell him now, his exams are a while away (assuming you are on an English timetable) and it gives him time to adjust, research and apply for courses.
I'd anticipate for him being annoyed and upset initially, and acknowledge how he feels and that it's unfair. Don't try and push him to feel differently. Reassure him that you want the best for him, and that you have confidence in him thriving in new area.
Once he's ready, I'd have some good points about the new area ready to share, and encourage him to go research more himself.
I think he will find something equally tempting to the football course. If it was a career plan it would be a different situation, but as he just wants to do something enjoyable for a year I'm sure he can find something good in the new area.
My concerns about new area would be about the Scottish education system being different to English and how he'd adapt to that. I'm assuming you can stay in Scotland for the remainder of his school education? As he's interested in military or learning a trade I'd research the equivalence of Highers to GCSEs if he ends up applying in England in future, to ensure he picks the correct options and aims for the needed grades.
There will be so many sporting and outdoors opportunities up there, I'm sure he will thrive there.
Can you contact any other military families with teens living there currently to put them in touch?