Polyamory

I've been in a relationship with this guy and it's just been him and I and another woman for the past few years. We both know each other and are cordial, but she and I aren't in a relationship together. Just with him. I want to start dating other people but I feel uncomfortable bringing it up to him. There's a guy that's interested in me and I'm interested in him. He knows of my relationship status and is ok with it. He just wants to date me and I'm interested in forming a relationship. I don't want to be dishonest to my boyfriend and do it behind his back. He's always been honest with me , I just feel like he'd be a little shocked because for so long I only wanted him and nobody else. Any tips on how to introduce dating other men to my boyfriend.