Mickey Rooney as 'Bill': stunning sequel to earlier Emmy-winning drama

Television has always been a copycat medium, which tries to emulate the success of one series by creating something similar. Well, now it is beginning to emulate the cinema pattern of creating sequels to successful specials. Just as ''Rocky II'' and ''Rocky III'' followed ''Rocky'' in the movies, now something that might be called ''Bill II'' follows ''Bill'' on CBS.

Despite its seemingly exploitative attempt to cash in on a situation already sensitively handled, Bill: On His Own (CBS, Wednesday, 9-11 p.m.) proves to be a gently exhilarating drama about kindness and sensitivity, about the human potential for love and understanding. It is so exquisitely handled that I find myself not only condoning the seeming crass commercialism which inspired a sequel rather than merely a rebroadcast of the original, but applauding the utterly successful audacity of the venture.

''On His Own'' is a kind of epilogue to the 1981 Emmy- and Peabody-winning drama which starred Mickey Rooney as Bill Sackter, a mentally retarded man whose courage and determination earned him a productive life outside the walls of the institutions that had held him for 44 years. When a friend and guardian of his moved to California, Bill found a home in town and a job on the campus of the University of Iowa School of Social Work, where he was befriended by another graduate student who undertook to teach him to read and write as part of a term project.

As portrayed in still another Emmy-caliber performance by Mickey Rooney, Bill is a proud and dignified human being, despite his handicap. The film traces his discovery of his religious heritage, his bar mitzvah at which he plays his harmonica, his problems at home and in his job, his relationship with the graduate student, the resolution of some of the normal problems of his life.

''After all that time we spent together, we never accomplished a thing,'' the graduate student cries in disappointment at Bill's inability to learn to read. ''But we got to be friends,'' Bill responds with agonizing honesty. His sense of ethics, his simple morality, have a kind of refreshing quality about them which may make some viewers wonder about our society's value system.

''You got to do your best,'' says Bill, ''that's a rule.'' And he does his best . . . always.

Skillfully and almost invisibly directed by Anthony Page with painstaking luminosity, acted by a nearly perfect cast with the incredibly moving Rooney always in control, the film is filled with moments of tearful jubilation. By the final scene, Bill has become a member of everybody's family.

At the end of this film, the real Barry Morrow (his friend and the writer of the story) appears on camera with the real Bill and the real Mickey Rooney. Then , a shocking legend: ''This film is dedicated to Bill Sackter, 1913-1983.''

''Bill: On His Own,'' by some sophisticated standards, could be labeled a simplistic tear-jerker. But it must be judged on its own humanistic standards, just as Bill had to be judged on his own. As such, it is a poignant and joyous example of television - and life - at its most inspiring.