Most students who major in math end up doing graduate study or working in related applied areas (you can see this in exit surveys from colleges), such as computer science, statistics, economics, finance or engineering. These students generally are served well by LACs, particularly if they combine math with another field of interest (which also will eliminate the problem of running out of math classes if they begin post-calculus).
However, students contemplating graduate study in math, with the goal of being a research mathematician (e.g. a prof at a research university), will be at a disadvantage attending LACs, even elite ones. The typical student entering a top math graduate program would come from (if they are from the US) a top private university or strong state flagship, would likely have started with sophomore/junior level math classes in college, taken honors classes, and finished with graduate level courses during their junior or senior year (in some cases they may start them even earlier). The research mathematician track is very competitive. Your son is likely only going to be on that track if he really loves math, and probably also only if he finds his math classes (even at a magnet school) not very challenging.
If that is the case, I’d recommend the usual elite private research universities (Ivies, MIT, Caltech, Duke, Chicago, CMU) and stronger flagship state universities (Michigan, Wisconsin, U C’s, etc…). Look for schools that have honors classes in math up through the advanced level (not just Honors Calculus, but at least Honors Analysis and Honors Algebra). These honors classes are designed for the relatively few students who are on the math grad school track. They are also small and taught by full time faculty (just as at an LAC). One somewhat less selective private school that provides these classes is U of Rochester.
The poster b@r!um went to Bryn Mawr, and ended up at an elite math grad school, but took grad level math classes at Penn as an undergrad – see the link below
http://talk.collegeconfidential.com/parents-forum/1414683-prestige-versus-cost-p7.html
Swarthmore also has advice which illustrates some of the difficulties students applying to math grad school from LACs might face:
http://www.swarthmore.edu/Documents/academics/math/grad_GRE/MathGradSchool.pdf