Math is better studied in college in the context of another subject that uses it (e.g., Physical Sciences, Engineering). Unless he wants to teach Math, and then you'd want to an Education major (and get a teaching credential) with a Math major/minor. Philosophy is also better studied in college in the context of another subject. Some majors are more "rigid" with the class offerings with a sequence of classes that starts Fall term of the freshman year, and if you miss that sequence or you change majors, then you may be on the 5-year plan.
If your son is in high school, have him meet with a counselor to discuss majors and career interests, and that should help guide college choices. Ideally you want to have a similar conversation prior to starting college with an academic advisor and a career counselor who could tell you what the job placement rates were like for (Math) graduates to get jobs in (Math) within 9 months of graduating and what the internships/co-op opportunities are before graduating. Ask about on-campus recruiting in a previous year with regard to companies visiting campus and recruiting new graduates in which majors. Typically it's engineering, computer science, business, and education being in the most demand. Graduate school should be a "plan B". You'd like to know that you have some solid career options when you finish the BA/BS.