Your son should definitely consult with an experienced NH criminal defense attorney in your area. Possession of a Controlled Drug and Transportation of a Controlled Drug are both misdemeanors with penalties that include fines and license suspension if he pleads guilty or is found guilty after trial. Your son could also face loss of any Federal student financial aid if he plans on attending college after high school.
Your son should absolutely contact a defense attorney. There may be constitutional issues about the detention, the search, and any statements that he made to police at the time of the incident or afterwards.
I provide free consultations via phone at (802) 318-8076 and often work in Southern New Hampshire courts.
It is always difficult to grasp the full extent of any legal issue from a brief introduction to a limited set of facts. If you are experiencing a situation that needs legal attention it is always best to set up a consultation with an attorney and go over the facts in greater detail. If you decide to enter into a professional relationship with an attorney hours of research will go into analyzing your case, so please do not expect that anything posted here should be relied upon without substantial further review.