Doesn't it drive you crazy when you miss something right under your nose? Argh!!! We have all been there, don't beat yourself up!
Needless to say, I would probably take your punishment a step further. I would not let him drive a car even to school until I have had proof through drug testing for at least 3 months that he is clean. I can tell you right now, the second he leaves for school in that car he will pull over somewhere and get stoned before and after school. Don't trust him for a moment.
I also would cut all communication from the outside world with him outside of going to school until he proves for a length of time that he is clean.
You can get your kids tested for pot or anything else easily and as often as you want. Me personally I would do it often. I know society today seems to downgrade pot smoking and now where I live we actually have dispensaries for medical marajuana all over the place. I could go on for hours on how stupid that is working out. Yet regardless of how minimal everyone seems to think it is, including your son, it is illegal and as a family you can get in HUGE trouble just having it in your house let alone being responsible for a minor carrying it around.
IT IS JUST NOT WORTH IT, but I obviously do not have to tell you that.
In short, all privileges over, no phone, no internet, no going out, no driving - period!
Drug tests at least twice per month and lessen as he proves he is sober.
This is your house and your son you are protecting, do not take it lightly! He will hate you for it now, you will hate you because it is a pain in the rear to have to monitor all this but you will not regret it in the long run.
Tell your son that as soon as he lives entirely on his own, you will drop it, but for now, it is your house, your rules, and pot is a NO GO!
Good luck girl!