Without being too presumptuous, your brother’s antisocial personality might best be investigated through a series of clinical tests. At 16 years old with these types of issues: aggression, stealing, poor results with education, problems in multiple areas of his life, difficulty with relationships, and being secretive, point to a need for understanding more about his motivations and concerns. At its core, this sounds like a learning disability that has become more difficult to rein in.
Children with Attention Deficit Hyperactivity Disorder are prone to a wide variety of issues all of which have been mentioned about your brother. While this isn’t meant to be a diagnosis; it is something I’d recommend you have a professional rule out for your brother. A good psychiatrist or clinical psychologist will know how to do the evaluation.
While ADHD may not be the diagnostic finding I think it begins the evaluation process. There are many other potential causes, each of which deserves to be considered and confirmed or ruled out.
The center for disease control had an extensive website on ADHD and special section on what they call other concerns and conditions that I encourage you to check out. Also, for treatment options, you can check out our own article here at PsychCentral.