son of an off-duty firefighter

The son of an off-duty firefighter was found guilty yesterday of resisting arrest and assaulting a police officer during a St. Patrick’s Day donnybrook that was viewed hundreds of thousands of times on YouTube.
-- Firefighter's son guilty in Metronews Vancouver, Wednesday May 28 2008

Is the father's job title, Off-Duty Firefighter? No, therefore start the article with, The son of a firefighter..., and mention later in the article that the father was off-duty. That fact is only relevant if the father was off-duty and involved in the donnybrook, which he was, so the opening sentence/paragraph (seen above) should make that clear.