NMG,
One thing I never enforced, even as a witness, was shunning my son who was dfd as a teenager. He was the oldest and his siblings saw my reaction to his situation. As teenagers, they never shunned him either. Now, as adults, they choose to be witnesses. They choose to not be close to him now, but neither strictly shun him. We have had them together at BBQ's.
One thing I have come to accept though, is that they each have their own decisions to make in life and I can't force everyone to get along. I have and still do let them know that I think shunning is totally wrong. They do lead very different lives and that is their choice. I see good in all of them and always will. I am their mother.
Your son may have treated you wrongly when you were dfd, and that is sad. He may have been following his mother's lead or been afraid of losing his friends, who knows? Taking a stand against shunning may help him see another side, but your sons may still choose to lead very different lives.
Grown children not getting along does not only happen to witnesses. I have sisters who basically shun each other and neither is religious in any way. What I learned from watching my parents is not to let any child force you to take sides. Let them work out their own differences, but love them all.
cl