Do you know for sure that he will get credit or advanced placement at his college? That is, will they give credit for his AP Calculus BC score? Will they give credit for linear algebra and discrete math even though they were high school classes (it might be easier if they were taken through a dual-enrollment program)? If not, will it be possible for him to take a placement exam?
Some schools have separate linear algebra classes for math majors and non-majors, and he might only be able to get credit for the non-majors class. This happened to me, but my school waived the requirement because I’ve taken higher-level classes in algebra and matrix analysis.
Another thing to note is that some schools (e.g. the [University of Michigan](https://lsa.umich.edu/math/undergraduates/courses-for-freshmen/honors-sequences.html)) have honors sequences for math majors where all the material is integrated together and the classes have names like “Honors Mathematics I.”
Multivariable calculus and abstract algebra are required for most math majors, so I would recommend taking those next. He can take them at the same time (which is what I did), but if he doesn’t want to do that, I would put multivariable calculus first because it’s probably required for real analysis. Differential equations isn’t always required for pure math, but it’s probably required for applied math.
As others have said, he will also be able to take classes in other fields of interest, e.g. computer science, economics, physics, and statistics. The philosophy department will have classes in logic, philosophy of math, and philosophy of science. Since he’s interested in math and psychology, you could also look into cognitive science.