In order to help your son to not only get admitted, but to thrive at a university in the UK, you need to completely reverse your thinking.
Don’t start out with swimming. Universities in the UK are not interested in hobbies. Oxford colleges don’t have pools, they have libraries. (A couple of Cambridge colleges do. You probably wouldn’t recognise them as such).
You have to start out with narrowing down the subject your son actually wants to study. Yes, the subject, because at UK universities, you study only one (a limited number of predetermined combinations are available, such as “maths and CS” or “politics, philosophy and economics”. You will have to check the websites).
And yes, he will have to do it now, as a sophomore. That’s what UK students do, then they tailor the subjects they study in their junior and senior years to their anticipated subject. In order to have a chance at an unconditional offer in early spring of senior year, he should tailor the APs he is taking, as much as possible, to the subject requirements, so that he can take all the AP tests required by the end of junior year, otherwise he will only get a conditional offer and may have to wait until summer and not know by May.
Once the subject(s) he wants to study has been narrowed down, you must check university websites to find out which AP exams they want to see. Most of them don’t even ask for your GPA, and they are not interested in classes that do not relate to the subject he is applying for.
If he feels he cannot commit to a subject in time to tailor his junior year APs to the requirements UK unis put on their websites, UK universities are not a good fit. That’s okay! US universities will be. It’s great you are starting the process now.