As a teacher, I find it quite amazing the nonsense spouted about drug abuse, because that's what it is. Any secondary school that says it has no drug problem is lying, there will undoubtably be some pupils dabbling, and they are easy to spot. They're the ones who underachieve, are unfocussed, can't concentrate, find it hard to stay awake. Now I was no angel, and in late sixth form and at college I dabbled in one or two recreational drugs, but dabbling was what it was. Kids experiment, that's inevitable, but there is a big difference between experimenting, and regular usage.
Regular usage causes IME major apathy, lack of achievement, and a lack of commitment. Now some may say that's your average 16 year old lad, it is but 10 times worse.
S and C you need to decide what it is he's doing. Is it occasionally for a laugh with the lads, and are they pretty decent lads (the stats say about 80-90% of boys will try recreational drugs by the age of 18) or is he hanging around with a bunch of wasters (smoke daily or nearly every day)
Which one it is will result in different actions. If it was the first I'd try to feign ignorance, unless it started to become the second. It's part of growing up, I went to the pub and threw up after 4 or 5 pints of Stella. I soon learnt, and didn't drink Stella! If he seems to be opting into a lifestyle, things he's always wanted to do like sport, music, university seem to going out of the window in order to hang around with people who are not a positive influence, that's when you need to intervene. Try and lessen his exposure to them, monitor his schoolwork, only let him out some nights, make excuses, (ask his school/college how they think he's doing, does it match expectations? They'll know if there's a problem.) Talk to Frank and ask for their advice, or speak to any of the appropriate charities. At least you are concerned, that in itself tends to lead to a positive outcome.
Good luck