Dr. Fred Bobb is a hematopathologist at Sacred Heart Hospital.
Apparently Dr. Bobb is very frightening because both J.D. and Doug are nervous to go into his office. Later, when J.D. actually does confront him, he seems very nice and perky. J.D. asks him to redo Ben Sullivan's bloodwork because he wishes it were incorrect.
Since the section of the episode where he appeared was part of J.D.'s fantasy, and in said fantasy it was the first time J.D. met him, it's likely that Fred Bobb never actually existed. This is further supported by the fact that J.D.'s original description of the hematopathologist was of someone who was terrifying, as opposed to Fred Bobbs kind attitude.