It would for sure be an asshole thing to do, but men don't really have anything to gain by getting a woman pregnant right? If anything it's a liability (child support payments)
Even if she leaves she'll have to continue to interact with him if he gets partial custody.
He can also threaten to make sure she has her child taken away if she leaves him and threaten to lie to the court about her fitness as a parent.
This works better if the local religion or government is anti abortion.