WASHINGTON — In strong, often personal terms, President Barack Obama on Thursday called for vigorous efforts to reverse underachievement among young black and Hispanic males. He also cautioned young minority men not to repeat his own youthful mistakes in an unforgiving world. The president kicked off his “My Brother’s Keeper” initiative from the White House East Room, appearing on stage with teenagers involved in the Becoming a Man program for at-risk boys in his hometown of Chicago. The aim is to “start a different cycle,” Obama said.