How are you communicating with him? Maybe he doesn't quite "get" that this is something you seriously want to pursue. Are you being assertive and saying directly that you want a poly relationship, or are you wording it more like, "what do you think about..." which might not seem like it's something you're actually considering? On the other swing of the pendulum, are you expressing yourself in such a way that he might feel steamrolled into agreeing to move forward, only to backtrack later because it wasn't really something he was ready for? Perhaps he's been holding back in expressing fears or concerns, or in saying what he really wants, during your conversations -- then, when it comes up again he needs to slow things down so he can be heard.
I'm not making assumptions in the above, just throwing out some ideas.
Another possibility is that it just doesn't seem like a real option to him. Do you have any poly groups in your area that you can connect with? Maybe taking him along to a poly group's organized gathering would help for it to really sink in, in his mind.