My best friend smokes weed, what should I do?

This is going to be long...please help
Well I'm in grade 12. I had this friend, he was a really great guy. We were close, as close as friends can be. I met his family slept over at his house like 50 times and our parents have become good friends too. I know probably all there is to know about him and I consider him more of a brother than even my own brother.
I hadn't talked to this guy for a month and he asks me if I want to go to an amusement park, so I say yea sure. We went together with 3 other of his friends, 1 of which who can drive. So we are going and we are hungry so we stop at this place to eat. We eat and my friend and this 1 other guy go to the car early. Me and these 2 other guys go back to the car to find my best friend smoking weed with this other pothead.
I didn't know exactly what they were doing... until I smelled the thing. Yea I know, I'm pretty white washed, w/e. I was stunned...to say the least. Like I was just like, what the f*ck... ... ... ... ... ... Like hes the last guy I'd expect to be smoking pot. So I was just like right, well this is probably normal for them so lets not let it faze me and just ignore it, so I get into the car. Then we go to some park and the whole lot of em starts smoking pot again... I'm just sitting there like I was watching a movie or something. Only thing going through my mind is like wow, what the f*ck?, wow some more.
I get out of the car and took a bus home.
I've been thinking about this for a long time.
I haven't talked to him in a year. I tried to forget him because if he wanted to do it, then w/e, he can do it. But our parents know each other, and this guy is my brother in everything but blood and I can't keep ignoring it, even it has been a year.
He sent me an email saying that I should call him because his family bought some presents for my parents while they were on vacation. So now I'm lost.
I don't know what I should do...and it seems like hes having fun too so who the hell am I to say whats wrong and wats right. Like he hasn't contacted me for a year, so that means that he forgot about me and is having fun living it all. But I am worried about him... it's grade 12 and he has to go to post-secondary... he failed civics in grade 11, like I know he isn't working as hard as he used to...
Should I just forget him and ignore him?
Should I try talking to him and get him off it?
Should I tell his parents?
Like wow, I've been thinking about this forever, and all answers seem like a bad answer... ignoring the problem won't make it go away, telling his parents will make him go farther away from me than he already is, talking to him like his mom doesn't sound that good either...
Ok and I'm not asking if you think weed is good or bad, I don't really care about that. I think it's bad, thats all that matters, I don't want your opinion on whether you think weed is better than cigarettes or something.
so please help! wasn't really sure where this would go under so I put it under relationships...and no I'm not gay >>

well,
my mom used to leave me at this apartment above us with a bunch of guys well she went out drinking and parting. one of the guys that lived ther did pot, he was only like 15. I was kinda young so I didnt really know what it was but I know it was bad. just a couple of weeks ago I saw one of his roomates and he and I started talking and I asked him how that one guy was doin and he told me that he died from a drug overdose a couple of years ago. just think, if you dont help your friend through this he could end up like that guy.
my mom used drugs once upon a time and look where she ended up. In jail and I havnt seen her for 5 years now. so at least talk to your friend and tell him your worried about him and you dont want to lose him. tell him exactly how you feel.

tell me how it goes. ill add you as my friend <3

you know, one of the people closest to me had similar troubles with weed, and it was so difficult to deal with. talking to him is probably the best thing for him and telling him your concerns. although, I'll admit... it probably won't work. I don't think you should give up on him, because eventually he can become the person he used to be. I gave up on a really close friend years ago because she did pot, and now we're good friends again, since she stopped. but losing the friend was the worst for me. telling his parents would probably anger him and lead to more drug usage. I'd say the best step is to talk to him about it...

you should really help your friend or he will end up were my dad is. hes been in jail for 12 years he just got out this year

 
There's a saying, "Friends don't let Friends drive drunk!"

That same sentiment applies to many circumstances. Friends don't let Friends do things that are potentially harmful to themselves or others.

Accordingly, as a Friend, I'd do whatever it takes to get him turned around. Even if it meant telling his parents. True, you might alienate him and lose him as a Friend, but, if you're really his Friend, losing his friendship is not as important as helping him gain his way back to a safe lifestyle.

I hope you do not take offense,
To that that's gone before;
'Tis only that it's my two-cents,
And not one penny more. §;o)