I'm not defending his behavior (disclaimer)
I am going to assume that this pregnancy was not planned. For a young guy, becoming a parent, especially if he wasn't planning on it, feels like the end of the world. He's faced with either giving up a lot of his ways and settling down, or giving up a chunk of his paycheck for child support. Either way he looses.
There IS a decent enough chance that once the baby comes and is an actual, physical thing, he may settle.