Other posters have provided a lot more useful advice than I ever could. So I'll be providing a useless perspective instead.
I have to say that your husband's timing is very convenient. It reminds me of my friend's ex-wife: She was cheating on him with an ex-bf (I'm not saying that your husband is cheating on you). Got tired of lying about her affair. So she poked some holes in a condom and got pregnant. She waited until the baby was a year old...then she proposed the idea of polyamory.
Now, in my friend's case, he emotionally disconnected from his then-wife (who, as you can probably tell, is kind of a bitch) and simply stayed for his child.
What I'm trying to say is that your husband is a dick. I know that calling him a dick isn't very constructive.
But he is a dick. A giant, flaming red dick.