I used to be in your husband's position. It took me several years to come around to the idea on my own, namely whenever I found a special someone that I wanted to spend time with.

If he is weary and uncomfortable about it, do NOT force the issue.

I assume you have copies of the various open relationship/polyamory books on hand? Open by Jenny Block, The Ethical Slut, etc? You probably do. Just know that reading about it, rationally discussing it is one thing, actually doing it and dealing with it is something else. And you both have to be in agreement before anything happens. You can't be too cautious, in my opinion.