Hi Greentea
I was just about to go to bed when I read this post and didn't want to read & run.
Mate, I totally understand your son. I was that person. I was the guy who played Warcraft obsessively. To the point where my kids knew not to disturb me and if I am honest, it played a large contributory factor as to why I broke up with my last ex.
You need to start treating this as an addiction. Because that is EXACTLY what it is. When playing your logic is compromised. You are in a world where it seems normal that everyone plays and those who don't simply don't understand. It's almost cult-like.
You already know why your son has become addicted. It's because in WOW, he feels like he belongs. In real life, people judge him and pass him off as a geek. A social misfit. In Warcraft he has the best armour, the best gear. People ask for him by name and he's relied upon, in fact NEEDED to complete raids. He's probably either a guildmaster or an officer of his guild. That means he's got some "authority" or power unlike in the real world.
In Warcraft he is rewarded for every little accomplishment, rarely punished. If he doesn't down that boss, farm enough materials for the next level item then someone else will. Over time he will be less important. An importance he's sweated 80 hours a week to achieve. And he's not going to risk losing his social status.
In short, he belongs.
I've gone on about this to hopefully make you understand what a powerful pull the game has on him. I remember in the height of my addiction. My ex used to turn up at the study stark naked and all I could think of was "FFS, now I have to go and I'll be letting my guild down!" 
So now you know why....the question is how do you break the cycle of addiction? You need something to replace it. The thing he's getting from WOW is social proof. What you need to do is discourage his playing whilst improving his social circle. Easier said than done I know.
If you can somehow get him to join some clubs, take up a sport, whatever. Over time you will probably find his interest wanes.
For me it was after my ex left. My kids needed me to be around more and as a result I went out more. As I socialised more, I found I didnt have time to play anymore. After a while I just quit as I was paying £9 a month for nothing.
Interestingly enough, after a couple of years I did go back to the game but was pretty laid back with it. Coincidentally I've just quit again today as I just dont have the time/inclination to keep chasing the epics and the amount of time the new expansion is demanding is way higher than the previous pack.
In the short term I recommend you limit his time to 2.5 hours a day and perhaps increase it on raid night (give him 1! Otherwise if he's hardcore, he'll be raiding 3-4 times a week). A limit of 2.5 hours will be enough to do an instance, farm some mats but not while away the hours on multiple characters.
If all that fails, you can invoke the ultimate sanction. Cut off the Internet. Actually or better still.....delete his characters. But you'd better be careful of the resulting explosion though.