the only charge he may get to stick is the interfering with an officer, but as the chief says he was dispatched to the scene as a firefighter. with out being there it just sounds like he got in the way and the police chief got a little butt hurt. he probably should have had a uniformed police officer escort him from the scene.
as far as the impersonating charge and practicing medicine without a license i dont think the DA will charge him with that. most all volunteer firefighters are trained on how to put a person on a backboard and most of them do not have a first responder license.
It would be a different story if he showed up and started and iv on the patient and said he was a paramedic. then it would be game on. I just dont see it sticking in any way.