Would you still consider Cambridge is better than Oxford in Maths?
But this is the thing ... better by what criteria?
If the single criterion is "top rankng" that's still not clear ... "best" for what? Student satisfaction? Staff research achievement? Employability? etc etc
But I think once your DS has dione a bit of research, and he's attended a couple of Open Days, he'll have more of a feel for where he wants to go. It needs to be about the course, the teaching style/s, but also all the other things.
Believe me, if students aren't happy in terms of how they're living, then they can project that onto their studies, and some of them end up not doing as well as they might.
Also, does he have any ideas about what he might want to do post graduation? Does he want to go into Mathematics research? Or into the City? Or teach? He doesn't need to be too specific, but if he has got ambitions in a particular area, he needs to look at degree programmes that will help him get where he wants to go.