And to check if a university is strong at the field, well, you can always look at the grad ranking for math as it seems to correlate with undergrad and/or look at the courses offered.
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Also, don’t be dazzled by the number of courses available. No. Make your son look at the possible math courses that he is willing to take. For instance, as a CS major who loves pure mathematics, I would have avoided almost any applied math/statistics courses offered by the university. This severely cuts the number of choices available for the student. And if your son gets even more picky and dislikes certain fields like Topology (which is pretty much geometry in a more abstract way), that school might not have as much math courses as it seems to have.
Sure what I just said sounds a bit “too picky” and very hard for many high school students to know, but it’s possible that he might already know what fields that he would like to avoid. For instance, during my high school, I knew I wanted to avoid any math courses related to biology, chemistry, statistics (and in college, even physics) and pursue computer science related math courses.
That being said, ideally, you should be broad about your choices with math. It’s just that there are students that are very picky and if your son is one of them, many liberal art colleges do not seem to be the best spotlight for your son. There are exceptions to this such as Williams, etc. etc. but it is something to consider. And oh ya, I think this pdf is worth mentioning for many aspiring math majors planning to go to grad school:
http://www.swarthmore.edu/Documents/academics/math/grad_GRE/MathGradSchool.pdf
There are some disadvantages with liberal art colleges in gettting to math grad schools. I’m sorry but even the math departments at Swathmore admits this. That said, I do not doubt the quality of education in liberal art colleges. It’s just that many professors do not know as much universities outside big research universities.