Can I join?
Fleur..my ds is 14 and we are having 'weed issues'! 
Over the summer hols he was going out with a girl who provided him with weed, through older friends of hers. He had actually told me whilst on holiday in Greece that he was 'going to try weed'
so I was really not that surprised when I found out he had smoked this joint. He had some left over from that occasion and stashed it in a tin under a set of drawers in his room. I found this whilst hoovering under the drawers and we 'had the chat' about the consequences of smoking weed etc etc.
I categorically stated that I never wanted drugs in our house again -
A few months later, he went to a party. He cannot drink alcohol (he's on ad's for depression) and to be fair to him, doesn't touch a drop. But at said party, he thought everyone else would be getting pissed so decided to purchase a splif for himself to have during this party so as not to miss out on feeling high/drunk etc. He had to store this at home for 4 days prior to the party so had totally ignored my strict 'no drugs' rule 
Well.. nobody drank! He said he enjoyed himself. But ruined the bloody moment by smoking the splif whilst hanging out of the window at his friends house when everyone else had gone to bed! Obviously they smelt it and the mother bought him back to our house (40 min drive away) at 7.30am the next morning 
Yesterday, I was searching his room (I feel I need to right now) and he had written me a lovely note under the drawers where he stored the weed saying 'fuck you'. 
He is, like your ds, a good kid at heart! He goes to a naice mc comp and is doing OK at school. It's just that he makes these God awful decisions and fucks up big time sometimes.
I have told him I won't lecture him. But I do cut out any articles in the paper I find that relate to the dangers of drugs. I forward stuff I find on the internet about the teenage brain/alcohol/death of other teenagers through drugs etc in the hope that in the future it might make him stop making such shit decisions. However, I currently have about a 10% feeling that this will help! The other 90% of me truly believes he thinks 'fuck you' and will do whatever he damn well pleases. Like your ds, he thinks we know nothing!
We found some old iPad clips of him when he was in Year 7 the other day. Playing 'spies' with his sister, using cushions to hide him. High pitched voice and lots of giggles….
Where the hell did that little boy go???