Since your relations are in tact you need to do the following:
$this->User->read(null, $id);
or
$this->User->find('first', array('conditions'=>array('id'=>1)));
And it should return also the courses for the user. It depends from your app, but I would use Containable behaviour to retrieve only the courses, but really depends how many relations you have with user's model.
Having Containable behaviour you should load only Courses relation like that:
$this->User->find('first',
array(
'conditions'=>array('id'=>1),
'contain'=>array('Courses')));