Re (from msue, Post #1
"In the same breath, the lady he has fallen for doesn't believe that he can love me and her at the same time."
This concerns me. The one thing this should not become is a contest where only one lady walks away with "the prize" (the man). Tell him about your fears that you might get "left behind" in this situation.
That having been said, polyamory has been known to work when everyone on board is willing to share. It's a group endeavor.
I hope your situation works out to the benefit of all concerned.