Marketable certificates? Depends on what your son is looking for. As a math major, he could almost certainly get a certificate in applied and computational mathematics (
http://www.pacm.princeton.edu/ugrad/overview.shtml) with minimal extra work, and if interested in finance or economics there is a Finance certificate (
http://www.princeton.edu/~bcf/certificate.htm), or in engineering management systems (
http://orfe.princeton.edu/ems/). All can be marketable if your son wants to work in such areas (quantitative investments are a good example of employers who love strong math majors with some finance). If just looking at grad school for math, I'd say that the ACM certificate would be useful and the others not quite so much.