The course work described approximates the course work that one would take as an undergraduate math major.
If he were to double major, he would have to plan his schedule very carefully, so that every course covers some requirement for one of his majors, or general education requirements not covered by the majors. Doing such a double major may be more difficult if the school has voluminous general education requirements not covered by the majors, or the majors have a higher volume of requirements (e.g. some schools’ math majors require additional course work like physics or CS).