Shawn's father definitely feels sorry for himself, although he does feel horribly that his son is suffering within himself. He is selfish and is more concerned about his own feelings and reactions than being there for his son or he wouldn't have left.  He leaves his son and the rest of his family because he simply "cannot handle" having a disabled son.  This, of course, reveals how selfish he is and how he is thinking of his own needs and not his son's best interests.  eNotes describes Syd, Shawn's dad:

Shawn's father is the other main character in the story, a driven, assertive man who is angry about his son's condition. Shawn's dad, Syd, is very intelligent and compassionate. However, he can also be seen as "weak and cowardly," which is how Shawn's mother describes him. He could not bear living with Shawn so he left his family. Syd cannot stand the idea that Shawn is suffering inside himself.