Welcome to our forum.
Sounds like your would-be partner needs to have some long talks with his pre-existing partner, figure out what he wants, what she wants, what he can live with, and what she can live with. He may need to slow down his relationship with you while working all this out.
I encourage you to do considerable reading and posting on this site, to get your own thoughts organized. It takes time and much knowledge to build a poly relationship and make it work.
Would his pre-existing partner be willing to read "Opening Up?" It might help.
Love means never having to say, "Put down that meat cleaver!"