the menu is not being displayed when the first parameter is user
From https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_menu/7:

"page callback": The function to call to display a web page when the user visits the path. If omitted, the parent menu item's callback will be used instead.

So page callback has to be omitted to fallback on the parents callback or has to be a function.