baby's last name?

I was with my ex almost a year, found out I was pregnant and when I told him he wanted me to get an abortion and he said there was no way he was having a baby, we broke up because there was no way I was going to end my pregnancy. Now I'm 4 months and thinking about names, he is now trying to put some effort, bought some baby clothes and always asks me how I'm doing and feeling. He brought it up the other day that he expects the baby to have his last name, but he hasn't even told anyone in his family I'm pregnant with his baby and he is always posting about his son on social media about how much he loves him and wants to be a great dad which hurts me that he's so loving but not to this baby. I'm not sure what last name to give my baby, mine or his. Any advice??