Marijuana is a gate way drug. This needs to be addressed right away. Look for Addiction rehabilitation centres. The most important thing to do is to find out the reasons for his use of drugs and work on this issue. It is important to address this use as a biopsychosocial model. Therapy and support from family members are major steps. Next try to get him to get involved in activities as if there is no free time there is no boredom and that prevents him from sitting and relaxing with the drug. Friends and family members along with AA/NA groups with a sponsor will make a huge difference. Try not to use a punishment model. Address the issue in a non judgmental manner and be willing to understand why there is a relapse and how as a support network we can help him fight the addiction. Try Marinnol if nothing works. Consult with your psychiatrist.