Seek support from the GP/SS again. It’s all you can do, maybe they will take it more seriously this time.
Speak to him regarding his path in life.
If he has left school then (unless he’s in a trade or something), he’s facing minimum wage jobs, next to no pension, and little enjoyment or security in life.
Young people will take drugs. I’m guessing you’re Scottish as you said “just now”, it is shit there, completely normalised for late teens to mid twenties to have ket and cocaine. Pretty standard to be offered either when you go to the bathroom in a bar.
I lived there around this age… the people who did ket and cocaine are not doing awesomely in any respect now and some are developing health conditions as a result of their persistent drug use… think bad enough to prevent them from working and providing for their family.. so a big deal.
Get him back into education and on track, it won’t avoid drug use but at least his life would be going in a semi positive direction. Make it a condition of his being in the house.
You could put him in a homeless hostel. But it’s likely to throw him from the frying pan into the fire in terms of contact with drug users and it will probably be harder stuff. Then you’re faced with years of back and forth with prison visits… so yeah don’t kick him out, keep pushing for help and getting him into a different environment/social group.