The student aid system is built around the assumption that undergrads are dependents of their parents and that the parents will be responsible for the bulk of expenses. So the answer to “how do they pay for the rest” typically is from the Bank of Mommy and Daddy.
If that’s not an option for your son, then here is what remains:
- Community college and/or local in-state public as a commuter (assuming that you are at least able to provide continued housing).
- The military -- either via an ROTC scholarship or by deferring college to to enlist in the military first.
- For high stat applicants -- a full ride scholarship, typically at a less prestigious state U. --but very high stat applicants might have nice offers even from private college.