Before you tell him, think about how you would feel if he told you a similar story. This will help you gauge the potential impact. When you start sharing, be as neutral as possible in your tone and description. Avoid any details that might make it seem like you are glorifying or interested in the cuckolding aspect. And throughout the conversation, keep emphasizing that your relationship is different and that you love and are committed to him.
First, make sure the story is really necessary to share. If so, start by prefacing it with the fact that it's just a story and has no relation to your real desires or expectations in your relationship. For example, you could say 'Honey, I heard this really strange story and I want to share it with you, but it has nothing to do with us.'
You need to choose the right time and place. Don't do it when he's already stressed or in a bad mood. Sit down with him in a calm and private setting. Explain that the story is something you came across and you found it interesting in a strange way. But also be prepared for his reaction. He might be shocked or even a little angry at first, so be ready to reassure him that it's just a story.