It may be marijuana, but it's classic addict behavior. He needs to deal with his probation violation, and should start doing AA or NA or whatever it takes for him to start taking responsibility for his life. If he goes into court with a sponsor, proof that he's going to meetings, a job, and a commitment to staying off of illegal drugs, he's more likely to get another chance. That presupposes, however, that he doesn't try to run and hide from this problem - he needs to deal with it.