The incident was reported at 8:50 a.m. in the 1100 block of South Harbor Boulevard, according to the Fullerton Police Department. The man was transported to a trauma center with injuries to his face and hands. He is expected to survive.
A sheriff's bomb squad responded to the scene and nearby businesses were evacuated. No structures were damaged.
An investigation indicated the explosion, which occurred near the side of the restaurant, was not electrical in nature as initially reported by a caller, police Lt. Jon Radus said.
Bomb squad deputies also located a second IED and rendered it safe, authorities said.
Charges will be sought against the injured man, according to police. His identity was not immediately disclosed.
Harbor Boulevard was reopened between Orangethorpe Avenue and Southgate Avenue after being closed for several hours.