I actually think when he does get sent to hell, he will become one of the demons that tortures the people who are sent to hell. That's what those tattoos around his body mean, i think. He will eventually become a demon
The story has done a great job at keeping the mystery of who the father is in the backround. At this point, in order to keep a element of surprise for us readers, the nature on whom the father is needs to hit hard.
The daughter's ex-boyfriend was introduced randomly and serves no real purpose...
The father of the baby is definitely her daughter's ex boyfriend. Given the timeline, he was probably still in high school when she did it with him i think. Which is why it would be problematic if anyone found out