I remember going through this type of fear as a child myself,brought on by my over active imagination.My parents used to make a point of checking out that nothing was hiding in the room,looking under the bed etc..It made me feel better,and that they were listening to me...They also kept a small light on in my bedroom,and showed me they were leaving the downstairs doors open,so they could hear me,if I needed to call them.Once I felt safe like that,the fears subsided really quickly.Hope your wee boy gets over this soon :-)