I just want to add here that it is probably important to try and find out what kinda weed smoker your son it. In my experience there are 3 different types of weed smokers.
The first one is the "chavvy" weed smokers, there are the kinda ones that hang around town centres, most of this group smoke it because they think its cool and most of their friends do it. Most weed smokers up until age 15 are in this group. Sadly this group tend to be the ones who get into trouble.
The second is the stoners, these are typically in year 11 or above. This is the stereotypical hippy kinda group this sums them up pretty well. However some of this group manage to do very well in college work somehow whilst getting stoned every weekend :0 . Some of the nicest people I have ever met have fallen into this group.
The third is the occasional smokers, which a surprising amount of teens fall into (I would say about 45% of a high performing sixth form), when out with mates or at a party they may pass round a couple of joints. Once again you only really get this group in year 11 and above. Most of this group are typically intelligent and liberals and will actually smoke weed responsibly and won't let it effect their education/job (However the law still punishes them for it, but that's for another day's post ;) )
If your son is in the first group then you will have more of a problem since he is probably smoking weed because its cool and to follow a crowd, rather than "choosing" to smoke it, and thus is hard to discuss reasons with him because he doesn't really have any.
If he is the second group then I would only really get too involved if it is effecting his college/school/job, chat to him about it and the risk, however I wouldn't start laying the law down otherwise he might start rebelling more.
If he is in the third group (which is doesn't sound like sadly) then I would leave it (unless it effects school/work as above) as he isn't doing anything abnormal.
In short the gist of what I'm saying is that not all teenagers who smoke weed are bad, so before you go in hammer and fist take a step back and think is the weed actually effecting the rest of their lives. If not then they are probably like 40% of people that age and thus punishing will probably do very little
My reasoning for the above comes from the fact that I know/have known people in all three groups (luckily less of the first group now) and have transversed many friendship groups since year 10 (including for a while some chavs including one who is currently going through the court process for posssesion with intent to supply cannabis) and thus seen pretty much all points of view on smoking weed that teenagers have and witnessed its effects on people (Which to be honest I have never seen it effect anyone badly apart from the first group)
(If your wondering I'm a 17 year old guy)