I would try to point out the times when things work out better and go more smoothly because of your friend. Maybe you could try to explain to him that you love him dearly and that hasn't changed, but you love it even more when your friend is also in the picture.
When you say that he's so traditional and you wonder how he deals with you, do you mean that you're very progressive and he's not?
Since you said he doesn't think about it ever, keep bringing it up. That way he will have to think about it and hopefully you two can make progress.
Have you brought up the idea to your friend? How does he feel about your friend?
I hope things go well for you, your husband, and your friend.