If a child is extremely afraid of dark, does that mean that he is going to suffer from schizophrenia later?

Unrelated generally. Being extremely afraid of the dark is generally unrelated to schizophrenia. Yes, if they are already seeing visions then it's linked, but more likely the fear is due to attachment issues, emotional immaturity, severe insecurity, or just plain overactive imagination.

No. Children often go through periods of night time fears and get over it--reassurance night lite etc might help also see if something might b making the child insecure.