I'm with those who take the least confrontational approach. By all means put him right where he's unfairly twisted things round by blaming you for his own actions, but don't attack Watchtower. Absorb his own assault on you and acknowledge his hurt. Remember Rom. 12:17-21 - the one about conquering evil by 'heaping fiery coals' upon your opponent's head? Appeal to the human, not the angry cult persona.
Suggested outline for a response, if it's not too late:
It was lovely to hear from him but you and your wife found some of his comments hurtful and unfair. You understand why he feels angry with you but he has no idea of the pain of what you and your wife have been through in coming to the uncomfortable conclusions you have about the Organization (do not elucidate!) or the anguish in making the decision to leave it. Those conclusions and decisions were not arrived at lightly, nor were they intended to cause him grief. They were for reasons of conscience and personal integrity (I can't remember if you still believe the Bible or not, but if you do, add something to the effect that you have to stay loyal to God and the Scriptures.) If ever he wants to know your reasons, you'll be happy to discuss them, but if not, that's fine also. It's important to have mutual respect of each other's choices. At this point, remind him of the Awake! quote about how people shouldn't be forced to choose between beliefs and family.
Reassure him of your love for him, his brother and the grandchildren. You miss the grandkids terribly and would love to visit with them, write them, Skype, send gifts, etc., however, out of respect for his and his brother's express wishes on [such-an-occasion] (remind him of what he said and when), you have not done so, as distressing as it is for you and your wife.
Then ask how the grandkids are. How are they doing at school? Have they been or done anything exciting recently? Say you've enclosed checks for each of the grandkids so they can buy themselves a treat. Ask that he conveys your love, hugs and kisses to the kids. You hope to hear from him again soon, lots of love, etc.