I am not all that sure that the Marines are or should be a drug rehabilitation program. Instead, you may want to pray for your son and encourage him to enter a rehab center and when he completes the program and remains clean, he can enter the Marines.
Even though we know that the military was never designed to be any form of a rehabilitation service, it has been for many. I know from personal experience that military training and experience can be the difference between a life ill spent and a productive existence.