Well, I think DOC has a point, although he might have stated it more tactfully. If you want to have a chance of enlightening your son you will not accomplish it by alienating his wife, that will just cause to then to refuse to talk to you. You also will not win by attacking his chosen religion. Part of the cult mind control is a warning that Satan will use their non JW family to test them. When that inevitably happens, it just confirms to them that the JWs are right.
The only chance you have is to feign interest, then raise questions that they cannot easily answer, or that causes them to think. The religion uses third grade logic and questionable reasoning, it doesn't hold up under scrutiny. You must educate yourself about how mind control works. You could say a friend told you the Watchtower has predicted the end several times, for example. He has probably not been told this. Of course, they have an answer for everything, but you might be able to plant a seed of doubt.