Should I give my baby my last name or the fathers?

So my first child has his dad's last name because we were together when he was born. He cheated and left and I'm pregnant with his child. He's with another girl and as of now he doesn't see the son he has and has no involvement in my pregnancy. I've been thinking about if I should just give me the baby my last name. The only reason I don't know if I should is because I don't want my kids to have different last names. So should I give it the dad's last name, give it mine and keep my son's the same, or give it mine and get my son's last name changed to mine.