I would never have thought of this, but here's one way to reassure your kid - sell his grandpa's ghost on eBay! Here's the auction if you're aching to get your hands on some research material (for those paranormal researchers out there - you know who you are!).
Update: The auction above was apparently not the original, but a copycat. And all the links are dead... here's a link to a news story about the original.