Make friends first. You don't have to tell him you are married before you even ask him out. That would be like assuming he's interested in you romantically. But you don't know if he is. Just say something like, "Hey, I always enjoy talking to you - would you want to grab a cup of coffee with me sometime?" But keep it casual and non-flirty. You might even add, "I'm always up for making new friends." Then, if he accepts, when you get together you can ask him what his situation is. "So, are you single?" See what he says. If he's partnered, find out if they're monogamous. Let that be the opening to explain your arrangement. And if it's totally clear that he's mono and there will be no chance of hooking up, you never even have to let him know you are interested in more than friendship, just carry on the convo as if he was a co-worker or neighbor you want to get to know - just friends. But if he seems interested and he's available, then explain: "I am in an open relationship with my husband, which means we're not exclusive. I'm free to date other people, and I was wondering if you'd be interested." Simple.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein