It's hard to know where the fear may be coming from without professional help. It could be something simple like he overheard a news broadcast or had a nightmare. It's very real to him and he needs validation for his fears. When my oldest was 5 my great-grandmother died. We were living 2,500 miles from home so we were unable to go to the funeral, so it's not like he had any negative memories from the incident (other than his grandmother not being there when we went home of course). He handled the news pretty well, but a couple of years later his grief was prompted after having a dream about my grandmother.