Just read the thread. Like some others, I encourage you to keep records of his contact with you, along with your responses. Print out the e-mails, save and print any chat sessions. That voice-mail to mp3 thing sounds pretty useful.
So, if and when you need to explain your side of it -- to your husband, his wife, or yourself -- the facts will be there for all to see.
People can do very stupid things while under the "spell" of infatuation. If he's a creep, well, then he is a creep and there is obvious danger. But, if he is otherwise a nice respectable guy, a family friend, then he has lost contact with reality and anything could happen.
Preserve and protect the facts.
Also, I noticed something in a couple of posts in this thread that seemed "ok" at first, but upon further reflection seems "odd" to me. It seems in situations like this -- where someone pursues another's spouse -- folks have gone out of their way to protect the would-be OM's wife from knowing about her husband's predatory behavior. Why is that? Seems like the wife would be better off knowing, in the long run. Seems like the wife would WANT to know.