A 16-years old and his father obtained a B1/B2 visa. There is a note on the son's visa saying "Vacation travel with father Los Angeles, CA". Do they have to fly to Los Angeles directly, or can they fly to e.g. San Francisco (and then drive to LA)?
This particular annotation is just a way for consular officers to pass information to the CBP officers at the port of entry. In this case, it's the purpose and location of the trip that was annotated. It is not binding on either the traveler or the CBP officer who will inspect the traveler at the port of entry.
This type of annotation is made under 9 FAM 403.9-5(G)(c)(3), which is simply generic information about the declared purpose of travel, which is intended to help DHS evaluate the person when they come to a port of entry or apply for an extension or adjustment of status. That section states:
A visa may be annotated in any case when you determine that the applicant is only nonimmigrant for the purpose of the particular visit for which the visa is issued. You may limit the visa validity in accordance with the guidelines in 9 FAM 403.9-4(C) above. Such notations will materially assist DHS inspectors at POEs as well as at the domestic offices of DHS and are encouraged. However, you may not enter negative notations such as:
“NO ADJUSTMENT OF STATUS or EXTENSION OF STAY RECOMMENDED.”
which questions the visa recipient’s veracity and/or tend to tell DHS what to do or what not to do in a given case. Under no circumstances should an annotation prohibit activity in the United States which would be permitted under the visa category, or prohibit the alien from seeking an extension or adjustment of status. These questions are appropriately the responsibility of immigration inspectors and the DHS.
As suggested by the section of the Foreign Affairs Manual it appears in, this type of annotation is typically used when the visa validity is limited for some reason. You didn't mention it, but I would expect to see that the father got a visa valid for 10 years (or by the the reciprocity schedule) and the son got a visa valid for 1 or 2 years.
The section of the FAM regarding limiting visa validity to which that section refers, gives an example which sounds quite similar to this traveler's situation:
For example, in a country with 10-year visa reciprocity, this could apply in the case a 17 year-old high school senior who has not yet been accepted to university and seeks to travel on a family trip with his or her family, when the consular officer believes the applicant overcomes 214(b) at the time of the interview, but would wish to re-interview the applicant in the future.