Canadian actor Brett Donahue is playing John F. Kennedy Jr. in the Reelz miniseries, “The Kennedys — After Camelot,’’ about the famous family’s life and times after 1963. The 30-year-old Donahue, who looked reasonably like JFK Jr. while on the set in Toronto Friday, is not well known, having appeared in just a few indie films and in small roles on TV. The miniseries also stars Katie Holmes as Jackie Kennedy Onassis and Matthew Perry as Ted Kennedy.