Varsity swimming and GPA will not do anything.
The only thing that matters is getting a 5 (4 in some cases) in subjects relevant for what you want to study at the university.
So, he has to decide whether he’s going for STEM or for Humanities/Social Science/Business, and tailor his AP’s toward that, then spend time to make sure he gets 5’s.
(He could also look at the AICE program or Cambridge PreU).
I agree with posters above; St Andrews, LSE, UCL, King’s, Imperial would be well-known and recognized in the US.
However LSE, UCL, King’s and Imperial are closer to US grad schools and are essentially what Americans would call “urban/commuter” (no “campus”, residences and students all over the city, etc.)
Durham, Queen’s Belfast are also worth looking into.
While not as well-known in the US, UGlasgow and Aberdeen (in general) and Herriot-Watt and Strathclyde (STEM, business) are solid and on the Scottish, 4-year system.
For something different, there’s also the ESSEC International BBA (business), Polytechnique’s Bachelor (Math&Physics), and Sciences po Reims (Econ, Poli Sci, History) - in English and in France, all offshoots from top-rated programs.