How to use user wildcard in a custom path?
You've picked user_load as your wildcard's load function - that takes an ID, and loads a user object. Your requirement is to load by username, which will work differently. 

A function ending with _load doesn't exist for that, so you'll have to create your own:

$items['user/%MYMODULE_user_name/job'] = array(
...

function MYMODULE_user_name_load($name) {
  return user_load_by_name($name);
}