It is possible that your father is attempting to transfer his wartime experiences in the direction of your 11-year-old grandson since he was the same age when he endured them. Although I understand his subconscious need to do this, your grandchild thankfully has a normal life and should not be subjected to his great-grandfather’s painful memories. His nightmares attest to this point.
Remember these points: Your father seems to be in need of healing; he is striving to accomplish this by finally sharing his stories. It is important that you do not try to stop him from talking about his encounters, only that you alter the audience. In this realm, consider suggesting having him join a support group or, if necessary, seek some psychotherapy in an effort to deal effectively with his issues. And do your best to make sure that he continues to talk about his painful experiences. This will increase the odds for success in his healing process. Hatzlachah!