How advanced is he in math?
If he is highly advanced (i.e. taking math courses more advanced than calculus 2 / calculus BC while in high school), then he may want to pay particular attention to how extensive the junior and senior level math offerings at each school are, and whether graduate level courses and research opportunities will be available to him as an undergraduate.
Is he looking at pure math or some sort of applied math (if the latter, what type of application)? Math is a fairly broad subject at the junior, senior, and graduate level, so checking how well the various subareas are covered may be useful. Also, if he is interested in applied math, offerings in the other relevant subjects of application may need to be checked.