Take the keyboard and mouse to bed with you!  That’s my first suggestion.  Help your son come up with a schedule to manage his time.  He shouldn’t have to do his homework after you go to bed.  If it is taking him more than a couple of hours to get his homework done (legitimately) maybe he needs to to the “computer/Internet” tasks first and then he can use the later hours to read or study other materials.

But there is a lot more to this.  Why is your son so attached to WOW?  Does he understand the long-term ergonomic effects that sitting at the game can have on his body?  Who is he playing with?  You may want to read my articles on Ergonomics and Multitasking to help you talk to him about what could be considered Internet Addiction.