Businessman John Laing launched a campaign for mayor Wednesday during comedy night at Slade’s Bar and Grill in Roxbury, according to a press release issued by the campaign. As mayor, Laing said, he would work to improve city schools, prevent youth violence, and support businesses that employ young people. “I am running for mayor of Boston because our city needs new and bold leadership in City Hall, and I am that leader,” said Laing, who lives in Hyde Park with his wife.