Maybe he is more sexual with someone new (nre sex is more stimulating?). Maybe he just doesn't get as sexified with you for whatever reason. Maybe he's just fine with the way he is and doesn't care if he functions or not.
Maybe letting go of the promise he made is a better way to go too. People promise all kinds of stuff when they aren't experienced in poly. At some point those early blundering promises should be seen for what they are and abandoned I think. If I were in your situation I think I would make my request that he at least slow down and then see if he considers it. If he doesn't then that to me is the measure of a poly man. I'd either keep at it and try not to freak out. Or move on.
Anyone want to be friends on Facebook?
Send me your name via PM