2B, you've been with this guy for 4 months and he's just now telling you he's poly!? Unless he discovered this about himself yesterday this is unconscionable to me. If it took him this long to mention something so fundamentally important I'd be wondering just what else he neglected to divulge. I'd also be having serious doubts about his willingness to share other important things as they come up.
It's good that you're reading and learning about how other people manage their poly relationships, particularly how monos handle a poly partner. There seems to be quite a few here. But none of that is a substitute for having a good, honest talk with your guy about what his vision for poly is, how you fit into that and what he's going to do to help you along.