WOBURN - Robert C. DeNapoli Jr., the son of the Woburn police officer shot in a jewelry store robbery in September, last evening raised his right hand before some 600 people and solemnly swore to accept the same duties as his heroic father. In doing so, DeNapoli, 27, became one of the Woburn Police Department’s newest members as a reserve patrolman, which is the first step in the ranks of officers at the Woburn police. “In the past two years there has been a lot of tragedy in the police department,’’ said Woburn Mayor Scott D. Galvin, referring to the shootings of DeNapoli’s father and the 2010 fatal shooting of another officer. “They have gone through a lot, and tonight shows our commitment to public safety,’’ he said. “It’s a good night for the city.’’