WASHINGTON, July 29 (Reuters) - The youngest son of U.S. Senator John McCain, a former prisoner of war in Vietnam and a vocal proponent of more American troops in Iraq, will soon report for duty in the Marine Corps, Time Magazine reported on Saturday. Jimmy McCain, 18, will spend three months in boot camp in California this autumn and another month in specialized training. Depending on his unit, the younger McCain could eventually wind up in Iraq where Marines have experienced heavy fighting, Time reported. Marines are also in combat in Afghanistan. "I'm obviously very proud of my son," McCain...