@Mefirst86 I think looking at this relationship properly for the next few months is the correct decision.
It could be all you have been through or it could be that the relationship is failing and you think the lack or marriage and a second child is a 'legitimate' reason to leave. Its obvious that these aren't the only things that are going.
I do think it's unfair to say he is calling all the shots. You said he has always been this way about marriage. If he always was, you chose to move in with Jim and have a baby with him. You did make your own choices about that, you did call the shots in your own choices
All he has done is communicate he doesn't want more children. Which he absolutely has a right to. It's not really him calling the shots. No more than you wanting one and leaving over it, is calling the shots for him.
He has always bene clear about marriage and now being clear about having more kids. You stayed and called you own shots over marriage.
Now you are calling the shots over kids.
I don't actually think you would be wrong to leave because I don't think this is an entirely happy relationship. But I think its wrong to position him as being the only one who is getting their way. You aren't a bystander in your own life.
Also, splitting is difficult for kids. But so is living in a house with parents whose relationship is failing and they won't face it. My personal opinion, is that splitting is usually damaging to kids because of how the parents handle it. Rather than because of the actual split.