D24 was struggling with a similar difficult decision - UIUC Grainger v. Washington University in St. Louis (WashU) for physics and philosophy. Ultimately, she decided on WashU for undergrad with an eye on UIUC for grad school.
There is a thread here with great advice: https://talk.collegeconfidential.com/t/uiuc-grainger-vs-wash-u-for-both-physics-and-philosophy-similar-cost
Best wishes to your son!