The first step is to confirm the child's fear. "I know you are afraid" this sentence is a seal of approval on the child's feelings. Never deny the child's feelings.
The next step is to talk to the child. Let him talk to you about his fear. Ask him if he wants to face his fear with you? Ask what can you do to help?
Assure the child that you are taking care of him. Lock the door together. Check under the bed together. Leave the bedroom and hallway lights on.
If he is afraid of sleeping alone, sleep by his bed for a while without holding his hand.
Never bring the child to your bed under the excuse of fear.
If the fear continues, consult a specialist