Josh was a boy from the dreamworld of the Nightmare Man.
Luke Smith dreamed about Josh. In the dreamworld, Josh completely replaced Luke. Sarah Jane showed him Mr Smith. When Luke called Sarah Jane from Oxford, she wasn't interested in Luke anymore. Sarah Jane, Rani Chandra and Clyde Langer only cared about Josh. Clyde, Rani, Sarah Jane and Josh burned all the things that reminded them of Luke. Josh only existed in Luke's dreams. (TV: The Nightmare Man)