That sort of question is a wee bit like asking "How long is a piece of string."
As someone upthread says, Cambridge is considered one of the best places for Mathematics, but is it the sort of teaching style, living conditions etc etc that your DS might thrive in? What kind of Maths? What does he want to specialise in by his final year? Which degree programmes offer the areas, options, and topics he's interested in?
And so on ...
With any generalist subject (just like English or History or Biology) you might start by looking at league tables for high research ratings and high student satisfaction ratings. These are not necessarily mutually exclusive as MN/general public mythology often assumes. Find maybe 10 degree courses/universities that might appeal, then look at their specific Departmental websites. That's where you'll find details of approach, curriculum, Department size, Joint degree programmes, emplyment opportunities, and so on.
Lots of good degree programmes now use FaceBook & Twitter to keep students in touch: you could search for those as well.
Then where does he want to live? How? City centre campus (eg Manchester, UCL)? Campus university (usually a greenfield site ... think of Lancaster or York).
And so on.