John Ritter plays Jim Belushi's buddy, gets recruited by the CIA and metamorphoses from wimp to macho man in "Real Men," which begins filming here next month--the second film under the new United Artists banner. (The James Bondian "The Living Daylights," which starts at the end of the month, is first.) . . . Rap music's the Fat Boys are hospital helpers in an old folks home trying to stop the murder of millionaire Ralph Bellamy in the comedy feature "The Disorderlies," directed by Michael Schultz. . . . Ed Asner is a convict who escapes from prison dressed as Santa and undergoes a conversion with the help of neighborhood kids in the Disney TV movie "The Christmas Star," to begin filming in Vancouver this week. . . . Ron Reagan Jr. shoots an episode of "Webster" Oct. 20 in which he's cast as the grandson of a World War II hero trying to stop the name change of Webster's school.