First, both you and your son must read The Teenage Liberation Handbook: How to Quit School and Get a Real Life and Education by Grace Llewellyn. Her second book, Real Lives: Eleven Teenagers Who Don't Go to School is also very good.
For information on the GED (Test for General Educational Development) try: www.gedonline.org. Students have used the GED like a diploma for college admissions, the military, or perhaps the motorcycle-mechanic school your son would like to attend. Good luck!