Never let it be said that success has gone to Billy Gilman's pre-teen head. Despite selling half a million copies of his debut album, the 12-year-old singer held court last week at the Washington Trust Bank's 200th birthday celebration in Westerly, R.I., just down the road from his hometown, Hope Valley. Gilman's duties included judging a cherry pie contest, singing the national anthem with a barbershop quartet, and leading the crowd in a rendition of "Happy Birthday."