I can vouch for the effectiveness of informational interviews because as a college instructor, I assigned them to my students. Significant numbers of my students narrowed down career goals, affirmed their aspirations, or learned that the career they thought they wanted was not right for them after all. Your son may not know what exactly he wants to do, but he probably has a few ideas. He can contact people who work in those fields and ask for 30 minutes of their time. He should make it clear he is not looking for a job at this time but would just like to chat briefly and learn more about the career field.
Informational interviews are a good tool.
Remember, however, that they should be set up with respect for the interviewee's time and position. Some job seekers abuse this opportunity by trying to find out if a company is hiring and for what kinds of positions. Limit the interview to questions about the occupation and why they enjoy it, not the company itself. Be sure to write a thank-you note afterward. Ask family and friends for contacts in the field you are considering.