Tell him you've posted it through the police station door and if he wants it back he can go there and ask for it.
My 20yo is a weed smoker and has been since he was 14. He was (is) an intelligent boy on track for 7 grade 7+ GCSEs until he started smoking weed. Nothing would stop him at that age. When he was 15 he was told he owed a local dealer a few hundred quid for his "free" weed and asked to do deliveries round town, and further afield (took a heavy, wrapped package that he thought had something metallic inside to Manchester). Eventually the penny dropped and he stopped doing the running. Just didn't turn up and they didn't know where they lived.
While all this was going on I had access to his phone and screen shotted every message. I passed it all to the police on a weekly basis, plus anecdotal material that he told me, and they used it with other intelligence they had to build a case and bust 2 local dealers. He was never named and never gave evidence or a statement, the messages alone were enough.
I got social care involved but they weren't great, TBH, although they did offer respite.
There is a whole lot more that happened - his behaviour at home got violent due to being unable to handle the drugs he was taking and he was placed in voluntary social care for 4 months for the safety of the rest of us. He was permanently excluded from school and only sat 2 GCSEs, both of which he passed but well below the grades he should have got. He's only recently picked up the threads of his education and is looking at an Access to HE course, but still only has his 2 GCSEs.
He still smokes weed but can manage it with a normal life. He has managed to get a decent job and a much better mindset. I'm waiting for him to get fed up of the weed, which he's showing signs of, but it's been a long road and it's had an effect on the lives of everyone else in this family, with two younger brothers having to see police coming to our house multiple times when he attacked me in a weed-induced paranoid rage, having their birthday and pocket money stolen to buy his weed, and listening to him cough and gag when smoking bongs in the middle of the night in the garden.
So those of you who bleat about it being harmless can fuck off to the far side of your your happy little hippy world and fuck off some more when you get there.