How advanced is he in math? I.e. what math courses will he complete before high school graduation?
Math majors who are highly advanced in math (college sophomore math like multivariable calculus and linear algebra before high school graduation) may want to take graduate level courses and do graduate level research, or at least take more than the typical number of upper level math courses, during undergraduate. So undergraduate-focused LACs’ and other colleges’ math departments need to be evaluated carefully to check for sufficient offerings for such students, if the student in question is such a student.
Also, “good STEM program” is too broad. A college can be good in some STEM majors (e.g. biology, various engineering majors) but not others (e.g. physics is often quite small as a major, and both it and math tend to have much instructional capacity consumed by service courses for other majors).