Two threads that you may be interested in perusing are:
Help find math heavy colleges for math head (jr)? (student who is a rising senior this year…and a resident of California, too!)
A place to study pure math— for the love of it (someone from the high school class of 2022)
There are potential schools but also some questions about math (that I am too ignorant to really know about) that you may want to consider as you go through this process with your student.