I am presuming that the hospital or the drug rehab centre where your son was under going treatment must have released your son to you as you are his legal guardian therefore in order to safe guard themselves they must have taken an undertaking that if anything were to happen (a relapse or any other causes) you will solely be responsible.
Lodge a police complaint with the local police that your son has had a relapse and that he is a threat to society and yourself and that he should be re-admitted back into the drug rehab centre.
This should solve your concerns.