There are lots of scary stories which you will find on the internet but they are too long.
So, I would recommend you an android app named "The Horror story:1k+ Haunted scary Ghosts Stories".
The application has many short stories. This short stories will help your nephew in the story competition.
Hope, this works!
Here is the links:-https://play.google.com/store/apps/details?id=com.ebizzapps.horrorstoryscaryghoststories"