Be honest with him right from the start. No one likes feeling like they've been deceived. Just say, "Hey, you seem like a cool guy and I'd like to see where this goes. You should know, I have a partner, X, she and I have been together for __ years. We just recently decided to open up our relationship, she actually encouraged me to get to know you better." Then answer any questions and clear up any misconceptions from there.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.