I'm going to second Magdlyn on this one, you should see a doctor first to talk about the problem.
There's also the distinct possibility that even if you do offer your husband the option you're still the only one he wants to be with. Up until I met someone myself fairly recently (and even then I'm moving very slowly with that) I couldn't imagine being with anyone but my wife even though the option was there. It may be the same for him despite his previous experience.
More than any of that, though, you should do it for yourself. Don't put yourself in the position of doing things you might not want to do just to make him happy at the cost of your own happiness. If you choose to go the direction of an open marriage, you need to do it because it's what you both want.