EX MAN See if this sounds familiar: A guy grows up in the Midwest, marries a television actress, moves to Hollywood, gets a messy divorce, and then marries a much younger woman. That is pretty much the premise for Tom Arnold's new series, The Tom Show, due on The WB this fall. ''It is a little autobiographical in that I slowly develop a relationship with a younger woman,'' says Arnold, 38, whose real-life wife, Julie, is 23. ''At some point the show will deal with that difference in age.'' But Arnold is adamant that the ex in the show is not Roseanne. Unfortunately, the well-known domestic goddess doesn't agree. According to Arnold, Roseanne is attempting to sue over the sitcom's premise. A spokesman for Roseanne will neither confirm nor deny the suit and has no comment on the series. In the interim, Arnold offers further proof that The Tom Show is pure fiction. ''We're talking with Sean Young about playing the ex-wife,'' he notes, ''and she looks nothing like Roseanne.''