I've been wondering this since a I saw a scene of him and Harley where he was asked to help give with a pregnant character. It was a funny scene as Harley questioned his credentials and by when he could not answer what kind of doctor he was she called him Mr. Fate. I've been left wondering ever since what kind of Doctor he is and how the title applies to him. Especially since Fate is Nabu using the helmet wearers as vessels if I'm not mistaken.