As the old song goes...Sometimes love just ain't enough.
Another way to look at it is that that when people find themselves in the situation where they really don't want kids and their really partner does, when one finally agrees to go with what their partner wants.
Yes, it is often framed in terms of loving their partner, but what I suspect is happening in the majority of cases is that it really has nothing specific to do with the partner, rather, it is the relationship they love. Subtle difference yes, but a huge difference all the same.
What it boils down to though for the people who shift their position, a significant part of their shift is fear. They fear the uncertainty of being without the relationship, they don't want to lose the comfort, the familiarity. Ending an established romantic relationship is a huge life change that impacts virtually every aspect of ones life.
It is clear that you are in emotional turmoil right now, and that has to be really awful. Just be prepared over the next few months especially to second guess yourself, for him to second guess himself. Be ready for your feelings to change when the desire to be back in the relationship gets triggered, and you want to get back together more than wanting to have a child. Be ready for him to be willing to have a child in order to reignite the relationship.
Just remember the sage advice of never making a life altering decision in the wake of a traumatic life event such as a death or a divorce, and this is no different. Ending this relationship is a life altering event, but whooooa boy, having a kid is even more so.