As you have already noted, it is really hard to keep a foot in both camps as it were for your high school preparation.
One option would be to pick out UK safeties to add to your UCAS list - and Uk safeties are real safeties, since admissions are straightforward (particularly if you’ve got your three APs in by junior year) and the expense is known upfront. Look at London schools and Scotland schools and Warwick for maths, @Twoin18 will be able to make recommendations I am sure. You’ll get 5 apps for the effort of one, and in the end, he may like his UK options best even if Oxford doesn’t work out.
You’ve already gotten great advice for structuring his math education. To emphasise the right mindset once more, Oxford profs will want to see maths, maths and more maths, oh yeah, our system requires a third A level, throw some physics or CS in if you must. As mentioned, forget stats, make sure he’s got those three AP 5s under his belt by junior year so he’s got a chance at an unconditional offer, then try to sort of recreate the further maths A level with the classes/competitions he has access to. If he is the person who thrives on that, UK universities will be the best fit.
Conversely, he may find that he does care more about the breadth of a US education. No reason not to try for Oxford anyway, but the way he his academic focus develops in high school will determine whether he is better off focusing on US or UK elites.