I am not a professional in mental health, but I do have some education on this. As others have mentioned, the best thing you could do would be to talk to a professional yourself, especially a professional who has experience with treating people with substance abuse disorders. Unfortunately, it is unlikely you will find a therapist who well-trained in treating other addictions, although they do exist. Often, therapists include friends and family in the therapy, and sometimes, they give therapy to just the friends and family if the person with the problem is unwilling to come.
Unfortunately, there is a lot of really bad advice on this thread. Do not hack his account, disconnect his computer, or any other majorly confrontational thing to take away the source. Do not get a bunch of his friends together and have a movie style intervention. He's addicted to the game; you will just piss him off and he'll never talk to you again and most likely find another way to play. Don't let others tell you that you shouldn't bother helping him because it's too much trouble; the very fact that it got this bad is a testament to how many people decided that it was someone else's problem, and the worst thing that can happen is that your friendship with him dies, and it doesn't seem to be doing so well in its current state anyhow.
Secondly, people only change when they're willing to change, which means that it's ultimately his responsibility, but that does not mean that you can't do anything to help him become more willing. Sit him down and quietly and politely express your concerns but try to engage him in the conversation. Here would be an example script, which I've adapted from the CAGE questions for screening for alcohol abuse:
"John, I've noticed that you've been playing a lot of that game lately, and I'm worried that it's not healthy for you. How do you feel about that game? Do you think that you should play less? Have other people told you that you should play less? Do you ever feel bad or guilty about playing so much?"
From that, you could determine if he's in the pre-contemplative stage (he thinks there's no problem) or the contemplative stage (he realizes there's a problem but isn't sure if he wants to change or doesn't know how). If he's in the contemplating stage, try to get him to verbally state the negative impact of his playing:
"It seems that you recognize that you are playing too much game. Do you think this is hurting your grades? Your social life? Your relationships with your parents? Your ability to date? Do you think cutting down on playing would help you in these areas?"
Once you've gotten him to admit that there are negative impacts of what he's doing, you've done a lot. From there, you should guide him into professional help:
"Well, it relieves me to know that you recognize that you have a problem. I'd just like you to know that as your friend, I'm concerned about you, and if you want help, I can help you get that help (i.e. refer to student health)."
That's really all you have to do. Even if he completely rejects you, you've at least planted some seeds of doubt in his mind. When he reaches rock bottom, those seeds will sprout, and you will have done the best you can as a friend for him.
Best of luck.