I just really don't think there's a whole lot else to discuss... I'm sorry, but I think it's best to just let this be as it is... You told him what you wanted and his wants differ from yours and that's totally okay but I think he doesn't need anymore explanation, but hearing more will only hurt him more. He knows you want to be poly and he doesn't so he did the best thing for both of you by saying goodbye. He doesn't need to stay with you and hurt himself by letting you do what you want and he doesn't want to trap you by staying with you and not letting you do what you want. Let it be goodbye and go your separate ways, I'm sure it hurts but it's for the best.

If you want to be friends, give him a few days and space to work through his own emotions, if he has any questions I'm sure he'll ask.