** Speaking as a mother of a 16yo ex-addict and something with a great deal of experience in working with vulnerable people.
You cannot punish someone out of an addiction. And you are mistaken to treat drug use as bad behaviour, it is much more complex than that.
Addiction, now known as substance use disorder, is a mental illness. People experiencing SUD need to engage with professional help if they want to recover. Buy in is essential just as it is with all other treatment. There is no sense in signing up for help unless you're prepqred to take it just as theres no sense in getting a script for antibiotics if you're not going to take them.
The best way to support your son is to do so firmly, with love. Be very honest about why his drug use worries you. Own your anxieties, don't just rant about it being illegal or foolish, claim your concerns and explain that your love for him is such that his drug use worries you terribly.
Ask him if he wants to continue to use or if he's interested in stopping. Ask what it is about drugs that he likes. Listen to his answers. Ask how he keeps himself safe ie. how he knows that what he's taking is actually what he thinks it is, how much he knows about his supplier. Ask what he needs to stop, what would make his life enjoyable.
Quitting drugs is very hard not least because it's very much to do with the company a user keeps so quitting means ending friendships.
Think about what you want. Do you want to see him recover and flourish or is it more important to you that he's out of your home?