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Were the two of them close? Is he not going to be able to see his grandfather because of the distance or because the grandfather is alienating himself from his family?
Thank you for clarifying.
When you speak to your son, it may be helpful to talk a bit about his grandfather's need for solitude. You can explain to him that some people seek that when they need to think about things, to regenerate their mind and body and when they for some reason had found themselves unable to stay close to those they love (versus being 100% unwilling)
It is possible that his grandfather is struggling with some kind of a mood disorder i.e. depression that is affecting his judgment and behavior.
If you have any contact info at all of the grandfather, you may ask your son to write him a letter of expressing his feelings. In the event that he replies, and has access to a computer, they may see each other on Skype or talk on the phone.
If there is going to be no contact whatsoever, you would be assisting your son in processing his feelings about this loss in his life. You can speak to him about grieving about someone even when they are not physically dead. He may have questions about why his grandfather is doing this. If you don't know for sure, you can let him know that, and list different reasons due to which a person becomes reclusive. Let him know that it is normal to miss someone even when their actions are strange or hurtful. Remind him that he can always talk to you about what he is thinking and feeling and that it will take some time to work on the sadness or emptiness that can result from all of this.
You're there for his emotional support and through your own example of how you deal with this, he can process the situation over time. It would be best to keep talk about his grandfather positive or neutral and not criticizing. Children are resilient and you're behavior/words will act as queues to him about how to handle this as well.