How about a little ceremony to politely ask the ghosts and monsters to move on? No harm in a little bit of acting if he has convinced himself they exsist and I think its sometimes best if we just accept our childrens fears and beliefs rather than try telling them its all in there imagination especially if its affected them so much they can't get on with a normal day. I wouldn't look too hard for a reason, theres alsorts on tv that tries to convince us ghosts exsist and he's probably picked up his fears from playground talk about the subject.