Can't help you with any past experiences of my own. wanted to send you a hug and say that I feel for you. Must be frustrating >>> MADDENING!
I can imagine 2 reasons for his acting like this:
1) He accepts the idea in theory, but keeps getting cold feet.
2) He doesn't accept the idea at all, doesn't want to have a show-down due to his saying a firm "NO!", and is using the back-and-forth as a delaying - or, really, evading - tactic.
Which does it seem like to you? (Or is there another reason?) What's he stalling on - that you have another lover, that you each have another lover, or that you form a triad with one other person? Have you got anyone in sight, or is this a case of setting the ground rules before the search for another?