@listener76 differential equations would probably be his first math course in college, since a lot of programs require it for math majors. Maybe real analysis and abstract algebra.
After that, it depends on what field he wants to go for. For applied math, he’d probably take some beginning applied courses (e.g. math for physics or math for economics). I am going more into theoretical CS and if he wishes to pursue a similar field he could take algorithms or automata/complexity theory which are fairly math-intensive.