When you say, 'I had planned to have DS take the AP economics to demonstrate he could do "college level" work... ', did you mean
- take the AP economic class? (It appears this is disallowed prior to 9th grade.)
- take the AP economics exam? (It appears this is allowed prior to 9th grade.)

College Board specifies:
Originally Posted by The College Board - AP Central - Appropriate Grade Levels for AP Courses
The College Board recognizes that there are some occasions in which students may be prepared to take an AP Exam prior to 9th grade. Because students are not required to take an AP course before taking the AP Exam, schools may choose to administer AP Exams to students of any grade level, so long as the restriction against use of the AP label on courses and transcripts prior to 9th grade is observed.

Students can prepare through independent study to take an AP exam. College Board provides course descriptions, lists of resources, old tests, etc, to use for study and test prep.

More links:
- macro
- micro
- exams

Homeschool kids can get a State photo ID, for admission to the test center.