Chibi Duckling
This whole thing is a conversation I dread having with him. But I know I need to do something before it gets out of hand. And... I do know how to be mean to him. I know just what I could say to put him off, because I know him that well. But it's totally a last resort kind of thing.
It's good to have it as a last resort even. As long as you have it as some option down the road it'll be good because this is all based on what ifs. The more important thing is telling him that you are uncomfortable with this roller coaster with him and you want to be just friends with him.
re-reading the OP though, he sounds like an a**. He wanted the open relationship just to be with that other girl and since you were having such feelings of jealousy, he decided to drop you for her. I mean, how long were he and this other girl not together before he started spewing the "I love you and want to spend forever with you"?