As I said before (and before), I’m not a math person. But if the Mathematics & Philosophy program at Oxford excites your son, here are a few other North American options that are looking to incorporate those fields (links lead to program descriptions):
- Yale offers a Bachelor of Mathematics and Philosophy
- Boston U. offers its own Bachelor of Mathematics and Philosophy
- Notre Dame offers a degree in Philosophy, Science, and Mathematics (perhaps broader than your son’s interests?)
- U. of Rochester has a number of allied classes between mathematics and philosophy and promotes the study of the two
- U. of Toronto has its own degree in mathematics and philosophy
- U. of Washington offers a math major with a philosophy option
- Wheaton offers a philosophy major integrated with mathematics
Also, have you seen this thread? It might be helpful: Best undergraduate program for philosophy/mathematics double major