I’ve heard similar stories of children seeing the spirits of friends and relatives who passed away before they were born, and knowing it was them without seeing pictures. So it seems almost certain to me that he is interacting with your boyfriend’s father. Children are more pure and in tune with their psychic abilities and the spiritual world– it is when we are older that we begin to tamp it down and ignore it. I definitely understand that this would be alarming, but I really see no cause for worry. Rejoice that your son is able to spend time with his grandfather!