My S25 is in a bit different position because he took college level math classes beyond what students typically take in high school, but it does seem like a number of universities we looked at would allow for students to test for placement into higher level courses, possibly by taking final exams for those courses to demonstrate knowledge.
I’m active in some homeschool circles (currently homeschooling my youngest), and I’ve definitely read stories about kids who self studied for higher level math and were able to test out successfully once they reached college.
Science might be more difficult due to lab requirements.
Are you open to online classes? That would be a way to get actual credit for the classes.