Should I give my son his dad's last name?

I got pregnant on accident, with a guy I've never even had a relationship with. He was weary at first, and angry about the pregnancy, but now he's accepted it and wants to be involved. I don't expect him to be an award-winning father; he's already got a son who he loves to death but doesn't do enough for. I'm glad he wants to be a dad but now what? Should my son have his last name? Or mine?