Be sure to check out state law/guidelines on that one, though-- where we are, you can't sit for a GED until you are 18. It's part of the state code on the subject.
Maybe an "unschooling" experience more like a job would be something to think about, though-- maybe a volunteer position with a local non-profit-- food bank, office, animal shelter, library, etc.
That might be a chance to work on the soft skills, executive deficits, and maturity. It does run the risk, though, of him deciding not to go back-- and the closer he gets to 18, the more that would worry me.
