When fear  of the dark  reaches a degree that is severe enough to be considered pathological, it is sometimes called scotophobia. Nights and darkness have always been associated with grim thoughts and beliefs.  This kind of fear of darkness or night is known as nyctophobia. This can take a severe form when prolonged to a long time period. The person can start avoiding any place which can be dark, or even stop night activities.  There are combination of treatment through exposure therapy, cognitive behaviour therapy and some medication (anti-anxiety or anti-depressant to reduce symptoms ). Please consult a Clinical Psychologist for necessary assessment and treatment

Has he always been scared of the dark? Detailed consultation with psychiatrist will be required to comment further on diagnosis. Anxiety disorders like phobia are managed with medicines and counselling.