Before the baby is born, you can't tell who the baby daddy is, only after the child is born.
For game purposes, the non-baby daddy won't automatically hate the child, all kids are ... neutral. Even sims who have the 'hates children' are oblivious to their own children being the thing that they are supposed to hate. That part is player controlled, so if you want step-daddy to hate the child, it is something you have to encourage.
What most likely will happen, baby momma will flirt with baby daddy, causing step-daddy to get in a bad relationship with the two of them.