A. I don't like secrets either. But I've noticed that not everyone shares my desire to know and tell. My advice is not to tell him -- there's really nothing to tell anyway, and your husband has to go on working with the guy. No reason to make the two of them uncomfortable with each other, and possibly spark animosity, over what (we hope) is a one-time idiocy on the part of Mr. Hot-to-Trot.
Try to handle the jerk by yourself. There are several ways to put him off your scent. Point-blank could work: "Your behavior is inappropriate, and I'm not interested anyway." Sarcasm and thinly veiled threats might also be effective: "Does your wife know how charming you can be?" The point is to let him know that this isn't going anywhere. Don't get flustered and say nothing.
Obviously, if his advances happen more than once or twice, you should talk to your husband. But if the guy stops, there's no reason to. Eventually, a few years from now, or when your husband or his amorous coworker gets a new job, you can tell him all about it, and the two of you can have a good laugh at Colleague Romeo's effrontery and high self-regard, and at his expense.