If he is very advanced in math (e.g. will complete college sophomore math like multivariable calculus and linear algebra while in high school), he should check that the math offerings at liberal arts colleges and other smaller colleges (including engineering-focused ones – at some, math is more of a supporting subject rather than a focus area) are sufficient for his interests (including graduate level courses and research activity that he will likely be interested in as an undergraduate).
Also, math at the upper levels is fairly broad, so some colleges’ math departments may be relatively stronger or weaker in some areas (this is often visible in course offerings and faculty rosters).