I think my real answer should maybe have been Other.
If he rolls the want again, I would have them break up. However... if it were me, I would have him briefly date the librarian, but continue to spend lots of time with your simself because of their child. Then eventually realize that the grass is not greener, and he wants her back. During this process, I would also have him work on changing the Erratic trait to something more responsible, through either Growing Together traits gameplay, or a combination of gameplay and cheats. Then he could work hard to woo your simself back.