[H69]

I think the main thing is to start slowly, don't jump in with both feet.
You could say you've seen a chatshow or womens magazine with women discussing having 2 men, and watch his reaction, you've known him a long time you should have an inckling of his thoughts. If he shows no bad signs, next time you're having sex mention the chat show, and tell him its been playing on your mind and getting you excited, don't say you want to try it, tell him its just a fantasy. Gradually build up to popping the question, but don't push any quicker than he wants to go.
From a male point of view, I enjoy sharing my wife with another man as long as his partner is there for the two of us (or more usually the 3 of us) to enjoy, but I would have to be very comfortable with the other guy to do just a threesome with my wife.
We are all different, just take your time, and remember we men like to think things are our idea in the first place, so see if you can lead him to suggest it.

ps Talk to your female friends. If you can find a like-minded one you can organise a foursome, not many men would turn one down if the wifes lead.