Your son sounds like a wise young man. It should be his choice to continue the relationship with his father and his grandparents. I would if I were you call the grandparents and let them know they overstepped their bounds with him. Should your son want to continue the contact , you can bet I would let him and the grandparents know it would be at my house or at a mutual place for all it would never be at their house again. Something tells me your son may not keep the contact with them them which for now may be in his best interest. You need to let your son know he did nothing wrong and he has the right to feel hurt or angry with his father, whatever the case may be there may come a day that your son will have a conversation with his father but it will happen when he is ready. It may not hurt to have him see a counselor in the meantime to help him through what is going on in his life. He just might need someone else to talk to that does not have family connection and can be on the outside of things. Good luck in whatever may come.