I think in most cases, most people grow out of it. I started smoking at about the same age, although i gave up about 6 months before taking my A levels because i was well enough informed about its long term use to know that it affects short term memory very badly and it wasn't a riski was prepared to take. My parents knew nothing. When i started smoking, my friends and i used to smoke it during lunchtime and i'd even go to classes stoned. I got 2 As, a B and a C for A levels and went to Uni. I did smoke the stronger strains, and took other drugs, while i was there. Tbh my smoking probably did get out of hand while i was there, but i still managed to get a good degree from a good university. When i joined the working world i continued for a while but then just grew out of it. One day, i realised that i didn't enjoy it any more - i couldn't join in conversations like i used to when stoned because i was too paranoid that people would laugh at me about my opinion. Instead of making me feel happy, i went into myself and became very introspective when i was smoking. I began to understand where my personal boundaries lay and cut down, until i stopped altogether. It just wasn't 'me' any more.
Out of all the friends i went to uni with, only one still smokes. All obtained good degrees (and we were probably the biggest stoners on campus) and now hold down excellent jobs. One is a lawyer. I have a good job with the police and have a 21 mo DS.
In most cases, recreational drug use does not equal a life of crime, drug dependency and under-achievement. That said, it's very much dependent on the type of personality, the group of peers who are also doing it, and the desire to achieve so it doesn't always follow. If your DS is determined to smoke it, and you have already spoken to him and told him you you don't want him to, the best thing you can do is ensure you're fully informed about it and illustrate to him the scientifically proven risks so that one day he can make up his own mind about whether it's worth it. Talk to people who have smoked about the effects, good and bad, so it's not just you saying "I've read in the newspapers" which most teenagers will sniff at in a "what do they know?" kind of way. You don't have to speak to him in a preachy way. Perhaps he has an ambition in life that you can draw attention to in a "be careful you don't jeopardise that ambition" kind of a way....
Good luck.