One of my favorites is "Husband, let me ask you -- what are your intentions regarding this marriage?"
You might also get deer-in-the-headlights, but I think it forces him to be a little more specific than the "work at it" one. He may, for example, a month and a half from now, in his DAM-ness say "Well, six weeks ago, didn't I invite you for a motorcycle ride? Wasn't that 'working at our marriage'??"
