Take a look at this Web site to find symptoms of marijuana use, particularly http://www.theantidrug.com/drug_info/ drugs_marijuana.html. I heard about a wonderful mother of four teen boys who met her children at curfew time under a bright porch light so that she could take a good look at them to see (and smell!) if they'd been smoking or drinking. Although kids complain about being watched, on a deeper level they experience our watching as caring.

Have a conversation with your son about the drugs found by your brother-in-law. First, listen to your son. What is he thinking or doing about his cousin's problem? Does he share your perspective that this is a problem? However, the fact that his cousin is using marijuana (even IF your son is not) is enough reason to limit time spent with his cousin. Drug treatment programs around the country have proven that hanging out with drug users is a predictor of whether kids will choose to do drugs too.

Whether or not your son is using drugs, something is negatively affecting him. Now is the time to explore why his grades have fallen and what you both can do about it. Your son needs to be pulled back into his own individual life, and he needs your help to do so.

Your "open dialogue" has broken down. Even if he is telling the truth, he has shown very poor judgment. He was driving with an illegal substance in the car. His cousin/friend was using this substance (while driving?) and endangering both of them. He is only a high school junior, he is new to these responsibilities and he needs more structure around his unsupervised time. He has signaled this need for structure by showing you that he is too young to make decisions on his own. Drugs are illegal and tragedies occur when kids use drugs and drive.