Choosing last Name!

So what made you ladies choose whose last name to give to your child?? The father of my child (Whom I am no longer with and do not want to be with) wants our son to have his last name but he has not really been around/involved for my pregnancy. We barely speak and I chose my son's first name. A little over a month ago is when he started checking on me and baby consistently and I am due in 5 days.  I don't want to simply give my baby his last name just because it's tradition but I'm contemplating whether my son should have his father's ln. Any thoughts? Did anyone regret giving their LO the father's ln?