My heart went out to you re: his leaving and not coming home. It's terrifying for a mom especially since he is only 16. It is interesting that he went to school though. I think you need to stand firm on the most major rules of the house which relate to his drug use. Tell him you love him too much to allow him to use drugs. I think the testing is a good idea. Is it a wide-spectrum test you are using? You really need to draw a line in the sand with this that includes regular and unsheduled drug tests with mandatory not going out until they are negative and stay negative. In a sense, you become the drug treatment, but I wouldn't do all of that without a professional to guide you. You said you saw Dr. Bradley recently. Seems like it's important to get your son to him ASAP for an eval. Maybe your son will agree if he sees it as a way to win some freedoms back?
One thing it sounds like your son does is throw around a bunch of statements to throw you off from the matter at hand. It's important to disregard all of that and not get into a debate. He's trying to distract you from the core issue which is that he's positive for drugs and exhibiting drug behaviors. I would think he knows in his heart that he is in trouble. It's not fun living like that. When my son was using he sounded the same and when he was in recovery he said that he talked like that because "I was what stood between him and his addiction/drugs"
Another thought is that if he runs away again, you could consider calling the police to look for him. THat might help in the shock it might be for him and if he were to be arrested he would most likely have court mandated treatment. But I don't know all the ins and out of that.
Bottom line, he needs an individual counselor ASAP and then let the professionals help with the next steps. We can't always do it all ourselves. His safety is the biggest concern. Hope you are holding up ok. It is very, very difficult.
"The world is full of story shapes."