Let him find you reading a book or magazine article about "swinging." Leave it laying around and he will definitely pick it up. If he's not a reader, he can find you watching one of the many documentaries about swinging. One of Dan and Jennifer's videos on this might be a great introduction. That will definitely peak his interest. That gives you a great "segway" into bringing up the subject. Don't just ask "Have you ever thought about __________?" If he has a fairly traditional upbringing, he'll say no out of fear of offending you. You may want to express interest and then ask him if he might be interested.
If you encounter resistance, you can always just ask why he doesn't think it's a good idea. It may just conflict with his values. He may be afraid of jealousy or losing you to another man. The reality is you will never know what he thinks unless you ask. Many, if not most, men fantasize about these things but never suggest them out of fear of offending their wives. From a man's perspective, I have something of a traditional upbringing and it took me a long time to realize that many women have these fantasies too. Your husband may feel he's respecting you by not mentioning it. Your reading a book or article or watching a documentary about it lets him know that it's your interest and gives him permission to say yes. If he has other concerns, the only way to find out is just to have this conversation with him.
Please let us know what happens.