By Abigail O’Leary & Abigail O’Leary Another 9/11 victim has been identified 23 years after the terror attack that changed the world forever. New York City medical examiner’s office identified the man as John Ballantine Niven, a 44-year-old executive at Aon Risk Services, an insurance firm on the 105th floor of tower two of the trade center complex. A father to an 18-month-old son on 9/11, he and his young family split time between Manhattan and Oyster Bay, where he grew up. Ellen Niven, his wife, said Thursday that she and their son Jack are grateful for the “extraordinary efforts” of city of…