We understand your concern. Cannabis can be detected for one month after last use.Sometimes test can be negative if marijuana for consumption was less frequent and concentration was less.It is better to motivate your son to quit marijuana as it is not good for health and it can lead to several Psychological and neurological problems. Consult a psychiatrist for the de addiction treatment. Doctor will initial give detoxification medicines. and later give an anticraving medications. He would also need to attend group supportive therapy to keep himself motivated. Hope the answer has helped you.