My suggestions:
1) Show him(them) something like what Scully posted above. Suggest for him to use logic in his thinking on this matter. If they claim the devil is behind all the wrong doings, then who is behind all the right doings and tons of love that is shown to man across the earth on a daily basis? Is then, that God? Is it God that is working in the background when hundreds of shelters each day in this nation, are feeding the homeless, and poor? Is it God that is letting a mother and father enjoy a great day with their children at a park, or is here there when they were giving birth to their kids which gave them such joy and passion in their lives? OR, ask him, could "Jehovah" God only be behind the happiness of the JW, and the rest of us it's just by chance. Point being, compare what they are saying about ALL badness in the world being Devil run, to who's behind all the happiness, compassion and joy. Would they have to admmit that yes, the devils is behind the bad, but God is not behind the good(but only for his organization)? If YES, then do they not seem like they are giving a foolish answer?
2) As others have said, and these were major points for me when someone was trying to lure me in....You have to convince them that there is NO WAY, God's people could have been started with such things as Charles Russell and the Pyramidology(show them the Pyramid Memorial), the embarrasing Beth Sarim incident( in the watchtower library I saw where they only say that the house was for retreat for Rutherford, so have clear evidence from somewhere that IT WAS bought for the return of the prophets and a failure of a prediction).
3) You have to have great info on the false prophecies, I have an Awake from October 8, 1968, that is sure to knock some sense into most New Generation Dubs. The months and years building up to 1975 as THE END, was clearly in many publications before 75, so try and get that together.
4) If they think and they are teaching that "touching Unclean things" is totally off limits and something their people try to live by, ask them if this is how God wanted to start his "organization", by having them associate with PAGAN "unclean things". In the beginning they had Crosses, celebrated some holidays, etc. It's one thing to "claim" over the years, they were just "FED" new light, but ask them..."Really, do you think that is how God would have started his organization?"
5) You need to make your son aware that if he wasn't with this DUB, how would he view all the things he's hearing from them?..... he would be able to use good logic and see the falsities of this organization, but he's in a situation where he's being influenced by a relationship.
Lavendar, I feel pain in my stomache for you over this situation you are in, and I've never been a witness. I am around these witnesses 5 days a week every week, so I see the mind control regularly, and I can't stand to hear you have to go through this...it just plain stinks!
to you, we'll all be thinking about you as you go through this, I'm sure.