Hi Caz, what a super aunt you are for being there for your young nephews when they most need you:)
In my personal opinion the worst thing you could do (which is seems very unlikely that you would do anyway) would be to outright oppose JW doctrine, as it will only make him grip on tighter. Subtleties are definitely the way to go, and he needs that you yourself used sound reasoning to discern that the JW's don't have the truth as they say. No question though that you're going to be in for a bit of a battle with his elder uncle, but please hang in there, he really needs you to fight for him. He's at such a fragile age himself, and he doesn't need to add to his confusion during this most painful time, so ease him out of it nice and steady. Since he is almost at adult age, he really does need to come to these conclusions for himself.
I think I could safely say that for many of us here the lack of "brotherly love", sincerity and a general not living up to the standards that we so believed in was the first clencher, and then working through the doctrine and reasoning through it was the second step.
Maybe don't go in too heavy at first, but the WT's membership with the UN has been a huge stumbling point for many JW's. I will post you a linnk to the United Nations confirming their association for nearly a decade until 2001.
http://www.un.org/dpi/ngosection/doc.htm (this link so you can trace its authenticity)
http://www.un.org/dpi/ngosection/watchtower.pdf
The heart of a JW's belief lies in their belief that the 'Governing Body' members are the 'Faithful & Discreet Slave' that Matthew 24:45 speaks of. There is only 1 mention of the FDS in the whole of the scriptures, for a free-thinking person it isn't difficult to reason that if this supposed FDS are intended to lead the little flock/great crowd here on earth during the "last days", that surely Jesus would have left a definite certainty of their authority, not merely an ambiguous reference to it in the new testament? If you allow him to read that passage of scripture for himself with an open mind he might be able to discern for himself that there is a definite ambiguity about it. Faithful & Discreet to me are merely adjectives, they're not intended to be taken up as a collective noun.
The most important thing once he has become convinced of these things to is to reassure him that no-one can take his belief in god away from him. The JW's as I'm sure you well know (as Christian fundamentalists) teach their followers that the only way to the "true god" is through them. It is this that gives them tremendous power over their parishoners and stops them from leaving. While I personally no longer hold a belief in god, the powerful social manipulation of the JW organisation is strong enough to convince and persuade you that to leave them means to lose a relationship with God/Jehovah. That is the part that is most difficult to bear, and the reason why the WT teaching is so insideously destructive.
I wish you all the best with your young nephews, and am very sad to hear of your loss. It does not of course surprise many of us here that the local congregation failed to provide the love, support and clarity that the boys father was promised he would get in his time of need. Not that I'm about blaming the organisation or its members, but many of us have found that our own conviction for right, justice and love was not truley replicated by the organisation as a whole.
Please keep us updated on your progress, you're got the right idea by doing your homework. Pls PM me anytime:)
luv from frog x