Have him go to Saxon's website and take the placement test. He might need to use the Algebra II book to work into the Calculus book, depending on how well prepared he is.
I look forward to the responses here.
If we homeschool again, I need to research on this as I have discovered that many schools (public and private) are not crazy about Saxon. My son said that what he is using at his school (private, Christian) is much better than Saxon. I will have to check the publisher on this when they get home.