Tell him that a careful examination of the history of the organization causes you to believe that it matches those Jesus warned not to go after, at Luke 21:8.
What you can do is read Luke 21:8 from the RSV and then draw to his attention that one of the early publications of JWs that contained many false end time predictions was a volume of Studies In the Scriptures entitled "The Time Is At Hand!"
Tell him you are very convinced that Watchtower matches Luke 21:8 and so you cannot in good conscience follow the organization when Jesus told his followers not to go after such ones. Here's a video worth watching to help you see what I'm saying:
http://www.youtube.com/watch?v=LSYBUifr7Y0
Expect him to misuse Proverbs 4:18 to give the usual Watchtower propaganda cliche about the light getting brighter. Research the real meaning of Proverbs 4:18 so you will be prepared to refute this tired, worn out cliche. You can also respond to it by reading for him 2 Timothy 3:7 and Ephesians 4:14 and telling him that those texts more accurately describe the situation with JWs.
Expect him to also use Acts 1:6,7 where the apostles are asking Jesus if he's restoring the kingdom at this time, to suggest that the apostles had wrong expectations and were not condemned and the situation is the same with JWs. You can respond by telling him the situation is not the same with JWs. First, the apostles were not yet anointed with the holy spirit - the spirit of truth - so they had a good excuse for not fully understand all things. By contrast, the JW leadership has always claimed to be anointed during the times they taught falsehoods. Secondly, the apostles did not go throughout Jerusalem confidently publishing a false message about the kingdom being restored at that time, No. They humbly came to Jesus and asked him a question. This is very different from Watchtower confidently publishing its false predictions far and wide.
Expect him to also use an argument like this:
"Well who else has the truth? Where else can we go? Who is Jehovah's organization today? Jehovah has always had an organization so, if not JWs then who else?".
This is meant to in effect say that you should stick with JWs just because you can't think of or find a better alternative and/or that even though JWs have faults they're the least evil of all the churches in error.
The way to refute this is to simply ask him which organization Jehovah was using during the centuries since the apostasy and before C.T. Russel's time. He would have to admit that there was no God-approved organization on earth that time. Watchtower said as much in a recent study edition studied a few weeks ago at the Kingdom Hall.
Once this point has been made you then tell him that since true christians existed among the wheat for all those centuries without there being a God-approved organization then it is very plausible that there is still no true organization on earth today - only individual true christians existing in the midst of wheat. You can then mention Jesus' parable of the wheat and the weeds and mention how the wheat grow together with the weeds until the end, that Jesus never mentioned the wheat being transplanted to a different field free of weeds and growing there for a while before being harvested. Jesus mentioned the wheat being removed from the field (which pictures earth) to the storehouse (heaven). This shows that the separating comes at the very end when Jesus judges the wicked and brings the righteous anointed christians to heaven.
Mention that an honest reading of the parable indicates that there will not be any true organization just erroneous organizations with wheat and weeds in them, and the separating of the wheat and the weeds clearly comes at the very end during armageddon/great tribulation. Jesus never mentioned a restoration before the end, neither did Paul, when he spoke of the coming apostasy and the man of lawlessness.
He will probably retort by citing texts in Daniel that talk about the righteous roving about and true knowledge becoming abundant in the time of the end. The way you refute this is simple. You just state that many individual true christians are getting an accurate understanding of the bible and leaving Watchtower and other false religious organizations. Additionally, since we don't know exactly when the end will come there's no way of saying for certain that that text is undergoing fulfillment in the present. It may still be a text that is to be fulfilled further in the future before the end comes.
Tell him that Watchtower is stressing organization while the bible is not and the reason why Watchtower stresses organization is because they want to keep themselves relevant and keep all JWs dependent on them and giving them their time and money to keep the organization afloat. The organization is twisting scripture to serve its own interests.
Tell him that contrary to what Watchtower self-servingly says, a hierarchical organization headed by a small group of men is not required to accomplish the global preaching work. Jehovah and Jesus can see every corner of the globe and know exactly where the message needs to be preached. And so they can do all the organizing in heaven and implement on earth by using angels and holy spirit to direct individual true christians throughout the globe to preach when and where they wish. This is what the book of Acts shows. Jesus used holy spirit and angels to direct the preaching work - not a hierarchical organization of men.