how do I ask for an open relationship

I've been with my boyfriend almost 2 years, we live together, he is AMAZING and treats me wonderfully but I just can't help but think being polyamourous would be fun. we have had a few 3 ways (me, him, and another girl) but I want to broaden it to extend to fun for me too, not just 2 girls for him. I don't expect him to want to have a 3 way with another guy which is why I think this would work out better. I just don't know how to bring it up to him. He has been in "open" relationships before where he has had 2 girls living with him and dating him at the same time but it never went the other way where the girls saw other guys. I want to ask him about this but I don't want to hurt his feelings and make him feel like he is not enough