Well I'd say there are many options:
He could major in math - and become a mathematician, middle school math teacher, high school math teacher, or college professor.
He could major in economics, accounting, physics, chemistry, engineering. All of those have a strong math component.
Seems like there are lots of choices here. You/he also needs to look at some of his other skills. Is he patient and good at explaining mathematical concepts to otehrs? If not he should probably rule out math teacher. Does he enjoy math enough that he wants to do it as a career? Or does he also have other interests?