TAPPAHANNOCK — A music producer who says he discovered Grammy Award nominee Chris Brown is suing the singer and his mother over a contract they signed almost six years ago.In the agreement dated March 12, 2002, Brown and his mother, Joyce Hawkins, engaged Warren M. Young as the “sole worldwide Exclusive Artist Manager” for Brown, who was 12 at the time.But Young maintains in papers filed in Essex County Circuit Court that he soon began “finding it increasingly difficult to make contact” with his clients. They eventually signed a management agreement with Zomba Label Group, a hip-hop and pop music company that also launched Britney Spears and the Backstreet Boys.