I work as a career consultant. There are two assessments that I would recommend for a teenager to help them make college major and career decisions. The first is StrengthsQuest by Gallup: http://www.strengthsquest.com/content/143780/Students.aspx
This assessment will identify where his natural talents lie, and give suggestions as to how to use this information in college and in a career. You will need to buy the book sold on this site (do not buy one used, since the code included in the book can only be used once to take the online assessment). The online assessment takes only 20-30 minutes and then he will get detailed feedback about himself and his strengths. The feedback is easy to read and to understand.
The other is called the Strong Interest Inventory. This assessment has been used in career counseling for decades. It doesn't measure skills or strengths, so it doesn't tell him what he is good at, just where he would likely fit in best in the career world based on his personal traits.
Here is a site that offers the assessment free: http://discovery.skillsone.com/slp.asp?adid=801&langu...
I would recommend reviewing the sample report first to get an idea what you will see after taking the assessment.
PM me if you have questions about my suggestions.
The other side of a major/career choice is job opportunity. I have been researching career demand trends, and almost half of the top ten career areas that are predicted to have a high demand into the future are Information Technology (IT) related. If he has an interest and aptitude in computers/math, this field is an excellent choice and will continue to be for the forseeable (over 10 years) future.
Again, PM me if you have more questions!