What notification am I required to give when starting an engine?
shouted as loudly as I could - PROP CLEAR - waited a few seconds and
  then started the engine.


Something to note here. While your shouting of PROP CLEAR was correct you need to Look Up and ensure no one is around when starting your engine. This is a common mistake, however you can bust a check ride for only shouting and not looking.  


  Was I supposed to walk up to him and notify him before starting the engine?


According to the FAA handbook, yes you should have alerted him and/or waited for him to clear the area. 


  Prior to engine start, the pilot must ensure that the ramp area
  surrounding the airplane is clear of persons, equipment, and other
  hazards from coming into contact with the airplane or the propeller.
  Also, an awareness of what is behind the airplane prior to engine
  start is standard practice. A propeller or other engine thrust can
  produce substantial velocities, result in damage to property, and
  injure those on the ground. The hazard of debris being blown into
  persons or property must be mitigated by the pilot. A