I used to be a regular weed smoker in my late teens and early twenties and I now have teenaged DDs. I have been honest with them about my drug use (and alcohol abuse, I have been in recovery for alcoholism for many years) and they are free to ask me questions about my experiences and for advice, and they do. We have a very open relationship, and at the moment, they both feel that they don't want to try drugs right now, but the older one may be interested in trying edibles when she's older. I think that my history of alcoholism (and that of my Father, and both of my Grandmothers) has shown them a family predisposition which both of our DDs feel strongly about avoiding, and so are planning to have a very tightly controlled relationship with alcohol (although DD2 has always said she never even wants to try it).
I wouldn't be socked or horrified if either of our DDs tried drugs or alcohol, but I would be surprised if they didn't tell me about it afterwards. They both know that they can call me any time of the day or night, no matter how drunk or high they are, and I will always be there to pick them up and bring them home. They know I wouldn't shout or scream (or cut up their debit cards), but would instead listen to them, and advise them on how to stay safe.
@sensiblesometimes, I think that once you have calmed down, you should go and knock on you DS's bedroom door and apologise for losing your temper. Explain to him that your anger was simply a result of the shock, worry and sheer terror you felt at finding out that he had been using drugs. I think you need to also apologise for going through his phone. At 16, I think he is entitled to expect privacy in this regard and you should reassure him that you won't do it again.
I would expect an apology from your DS for his (over)reaction in punching a wall - violent behaviour (even when aimed at inanimate objects!) is never acceptable.
I would hope that from these apologies, an open conversation would follow, and your DS might be able to talk to you about how his experimentation came about. With some more knowledge, you should be able to set some new ground rules with him, particularly in reinforcing a curfew, agreeing that he needs to let you know where and importantly, with who, he will be when he goes out. I also think that letting him know that whatever sticky situation he gets himself into, you will always come and get him, will reassure him (and you).
I know it's a scary situation for you and that it must have been a HUGE shock when you first found out. I know you want him to be safe, but if you can get him to open up to you, you are much more likely to be able to influence his choices into more sensible ones and knowing that he can tell you things without you stopping his allowance and cutting up his debit card(!) should encourage him to be more frank.
Go and give him a hug.