Hi Parent SW11, I just wanted to say that I really applaud your attempts to get together with other parents who are in the same situation. My children are still in primary school so fortunately are not exposed to the cannabis-smoking peer group yet. I have sadly seen first-hand the damage that regular smoking of cannabis can do. I'm not talking about the odd smoke, but the almost regular daily use that seems to be sadly prevalent amongst some teenagers. My mum's friend has two twin boys who started smoking cannabis heavily at sixth form. They went from A* GCSE pupils to complete drop-outs. Fast forward 10 years and one is now sectioned in a psychiatric hospital and the other lives at home and is like a zombie (to use his mother's words) who sleeps all day, and has no motivation to get dressed or washed, never mind think about looking for a job.
I know that cannabis smoking does not always cause psychosis and for some reason some people seem to be more susceptible than others. Having said that, I tutor a teenager, who I know is smoking cannabis on a regular basis, and sadly I can see that quite often she is just not with it - the lights are on but there's no one at home.
I really hope that you get support from the other parents in your situation. Is your GP or school offering any support? Presumably his teachers must be aware if he has only sat two out of the 10 GCSEs he was due to take? Please do not lose heart, there will be help out there for both you and your son. And the important thing that you're doing is to try and tackle the issue now and get some good advice from others - professionals and those who have been in your situation.