Without knowing any of you personally, may I add some thoughts....
40 hours per week, despite what people say, is a long time to spend playing computer games. This is a full working week for a lot of people, and 6 hours per day involved in WoW alone is certainly overkill for what is supposed to be entertainment.
You say that your brother is quite a loner and is certainly rejecting the idea of counselling very strongly. Well, TBH I don't blame him. If I may spend a few minutes explaining myself - please correct me if I'm off the mark.
1) Your brother doesn't find much acceptance at school, doesn't have many friends and is reluctant to engage in social activity. Yet in WoW he finds himself engaged in a totally self-controllable environment. While he may not be able to change his physical appearance (should he want to, of course) or make ppl like him outside the house, within WoW he is in control of every aspect of his actions. He can effectively look the way he wants to look, do what he wants to do, and interact with ppl whom he knows have a kindred spirit. He feels his responsibilities lie within this artificial world - people rely on him here and he pulls through for them when they need him, within the realms of the game. The feeling of connection or belonging which he may lack in the real world is met within WoW, more intensely than it ever could be outside. After all, he gets to be a fantasy hero/villain outside the constructs and barriers of real life.
2) He shows a quite apparent case of denial and delusion. I don't mean this in any kind of insulting way, merely that his life inside the game has effectively replaced his life in the real world and he does not see a problem with this. In fact, there is no problem with diving into a fully realised virtual world, per se, except for the severe damage it could potentially do to a person's psyche if taken too far. If I were to advise your brother (should he ever ask it) I would suggest that he examine what it is he finds purpose in within the game and try to apply this to his life. For example, he takes great pride in being relied upon for raids and performing tasks, because he believes that it is important for him to live up to the expectations and needs of others. This feeling can be transplaced into the outside world in many ways: getting a job, joining a club, etc, all kinds a banal examples - but the outcome is the same. Your brother needs someone to need him. Without this, it is unlikely that he will be distracted from WoW, where he has a multitude of ppl who rely on him for certain things.
3) I applaud you and your family for realising that your brother has gone too far. Certianly, such a time focused solely upon one aspect of your life is going to be a major influence upon your general every day actions. For those of you who have the time, perhaps 40 hours per week does not sound too long - but for a person at school or university, this is far too much. He will physically not be able to continue his studies - and doing well in school is something that your family should be placing as number 1 priority for your brother. As far as 'dealing' with him goes, may I make a couple of suggestions from my experience:
-your brother does not see his actions as 'wrong' (they are not 'wrong', merely excessive I think) - therefore it would not be effective to approach him in such a manner. Do not make him feel as though he is in trouble for misbehaving, etc - his WoW virtual life is far more interesting to him, so do not belittle his behaviour as this will effectively belittle HIM. And no-body like being belittled; you will get little, if any, effective response.
-remind him of his responsibilities. To his family, his friends, and to himself. Do not make them sound like a chore, rather allow him to see that his greatest potentials lie with those who care about him personally.
-don't impose restrictions on him. If needs be, get him his own computer and ask him to reduce the time he spends playing. Approach this as though he were doing YOU a service voluntarily, not that you were forcing him against his will.
-don't force counselling on him. I know that today's culture is very therapy-friendly, but the problems your brother faces are not easily discussed - especially for someone his age. Counselling is scary for anyone. But, take solace in the fact that, because he is so scared of it, it is likely that he himself has at least sub-consciously realised that he has a problem. Were he 100% convinced that he wasn't doing anything wrong, I doubt he would have had such an aversion.
4) Remember that your brother is fast becoming a man (I seem to remember someone saying he was 17?). As such, any decisions relating to his personal life should be made by him and him alone. This does not mean to say that you can't try to convince him. merely that you shouldn't force him.
5) He has an addiction; and, as such, your approach should be similar to that of someone suffering from any kind of addiction. They say the first step is acceptance, and they're right. Once your brother realises that he is addicted to the game, he will be in a position to change. You can't force him to feel this, merely help him. You shouldn't advocate 'cold turkey', but try to come to some kind of compromise - try to get him to reduce his gaming slowly over the summer - maybe get him a job or something - but make sure that HE is the one who makes the decision to cut down - it won't be nearly as effective if he is forced to. Any one tried to give up smoking? It only works if you WANT it to.
6) Remember he is your brother and you love him. Don't make him feel any more outcast than he might already be. Engage with him about it, try to relate it to your experiences, and remember that it will take a while.
I hope everything works out for you,
Nick