I feel for you, OP. I went through the same when my DS was 17. Disappearing, taking drugs, dropped out of 6th form, fell in with some local dealers who were the same age as him.
Life was hell.
I dragged him to a drug charity for under 25s, he was brazen about what he was taking. The charity were very forthright with him.
I called the police every single time he went missing. Every time. I got an alert thing (forget what it's called) put on the house so the police always came immediately.
I discovered that drug deals were being done through my letter box during the night so I sealed up the letter box and put up security cameras.
I called social services. They were bloody brilliant and worked with us all as a family. They helped set up a list of expectations on both sides (him, me and his sibling) and we all signed it as we would a contract. He was told, in no uncertain terms, that if he didn't get a full time job, stop the drugs, start being respectful, then he would be leaving the home for good on the morning of his 18th birthday. Social services described in detail exactly which hostel he would be living in and their rules which were far stricter than mine.
With all of us working together, he dropped the dealer 'friends', stopped the drugs, got a fantastic job and literally turned his life around.
I couldn't be more proud of him.
He's 25 now, has his own house with a mortgage, fabulous GF, his current job is brilliant and he's been promoted several times.
It's a horrible, horrible time, OP. Reach out to every agency you can for support and I truly hope your DS's outcome will be as successful as mine.