If he really loves you....sigh.
If you don't want the gf around I think that is within your right to have that. However, there are tons of ways he can accommodate his NRE - however ...I don't know why you'd want to stay with somebody who doesn't know how you manage NRE.
Maybe you become friends with the girlfriend and she will start watching the kid for a few hours once a week so you can have a date with your husband (I mean months from now). Your husband arranges with your mother to watch the kid for a few hours so he can take you on a date.
Your husband makes a schedule with you and the girlfriend so he can go have some time with her but you know when he is going to be home, and he is home doing what he is supposed to for his child. He is supposed to be negotiating time, energy, balancing his responsibilities.
Did he actually read any books or forums or ANYTHING about poly yet? If his idea of poly is he just goes "omg im in luv!!!" and then he gets to abandon everything because he needs to bask in the glow of new amazing shiny thing... Books and websites (in the sticky thread on this forum) talk about this shit, including how to manage NRE. If he doesn't figure out managing it is necessary, you wouldn't want to stay with him anyway, as he will just act like a major ass anytime he gets a new interest.
The question isn't should he wait for a year to see if you will change. Maybe you do have changing to do, and you should be doing that, but if he thinks his behavior is acceptable, then he is years away from realizing he has some growing up to do himself.
Sorry, this might not be useful advice, I am livid at what you are saying about him and feel like ranting and hitting him in the head with a frying pan myself, so I am not sure how you are not putting yourself first and taking blame for anything at the moment.
Happiness will never come to those who fail to appreciate what they already have.