I commend your son for wanting to acquire a better education!
Public schools tend to provide education to children, if needed, until they are 21. However, most states have compulsory attendance laws, which students are required to follow. If your child is willing to return to school on a regular basis and follow the school's structure and class routines, then contact your school's director of guidance and counseling to see about enrollment.
If the student has been out of school for more than a semester or two, many young adults are not comfortable returning to a school's structure and routines. Some look for alternative programs, such as high school at night or a GED, an alternative test for the high school diploma. Consider calling your local high school for more information. Or, check out the attached web site to give you more information about the GED test. Plus, there are many more articles to explore on Education.com.