JEM – I’m sure drugs can be bought everywhere , your son does sound depressed and needs encouragement to come out from his room and see daylight.
If I thought my son was getting drugs delivered I would lock the doors and keep the key with me , leave your bedroom door open in case you hear them being dropped through the letterbox or windows opening.
Your sons sleep pattern maybe up the wall and he might be watching films all night , so tell him giving him warning that as from such a date it’s best that no television should be on after a certain time except at weekends .
Tell him you love him and are worried he may be unhealthy if he hibernates in his room all the time.