I can't tell exactly how old your DS is, but I'm guessing around 9 or 10. I have heard of kids as young as 6th grade taking AP exams - but I can't think of many reasons why this would be a useful thing to do.
If you've already signed him up for the AP exam, please ignore the next two paragraphs. You say you had planned for him to take AP Econ (Macro or Micro, or both?), but if you haven't signed up for it yet, you're likely too late for this year. April 14th is the final deadline for schools to order exams, but most schools have already ordered the exams. Also, it isn't like the SAT or ACT. If you aren't a student at a HS that gives a particular AP exam (or at least a student in that school district), you need to find a school willing to let your kid take the exam. And schools may be hesitant to let a very young kid sit for the exam. I've heard that homeschoolers of HS age sometimes have trouble finding a school that will let them sit for an exam.
APs are only given once a year, in May. Scores are released in July. So if you've missed the sign up for this year, you'll need to wait a year to take the AP. Also, I don't know how comfortable your son is with writing essays under time pressure, but most AP exams, including Micro and Macro, involve writing several short essays in a short amount of time.
Could he take some lower level econ classes at the local U, even if he knows the material, to show he can do the work? Maybe take a summer class or two, and contingent upon performance in those classes, be permitted to take higher level classes next year?
Also, I'm not sure of the rules regarding FBLA events, but this might be a way for your DS to show what he knows. I know they are intended for grades 9-12, and the middle level FBLA events are different from the HS FBLA, but perhaps a homeschooler could compete as an individual in FBLA - something to check into.
