Your $user does not return a \App\Photo model. So if you do dd($user->photo) you will probably see null on your screen.
Does the user you are trying to view at this moment have a photo attached to it? Take a look in you database table user or do: dd($user->photo_id)
Relationship
I think you use the wrong relationship here. How does your database look like?
Does your user table have a column named photo_id? Or does your photo table have a column named user_id?
If the last is the case you should use hasOne() relation instead of the belongsTo() on the User model. See for more examples the documentation
on one-to-one relationships. https://laravel.com/docs/5.5/eloquent-relationships#one-to-one
App/User
public function photo()
{
return $this->hasOne('App\Photo');
}
App/Photo
public function user()
{
return $this->belongsTo('App\User');
}