I've only lived in one house with a ghost. I saw him at the head of the stairs once. My young son saw him all the time and was frightened by him. I did some digging and found out that the man had committed suicide in the house. I was never frightened.
I hate that your son was scared of him! I have never seen a ghost but had some weird experiences- although I am not sure they were just evil and not harmless "ghosts."