Ladies I need some opinions. So me and my ex are getting a divorce, we have a daughter and our son…

Ladies I need some opinions. So me and my ex are getting a divorce, we have a daughter and our son I’m pregnant with. He has never grown up and never really cared. Today he asked his mom if he signed his rights over if he wouldn’t have to pay child support, he is also dating an underage girl and told his dad my son wasn’t his to make him not be mad then got mad at his mom because she told him it was, like how can you deny your children? But anyway she has his last name and I don’t want him to but I also don’t want them to have different last names so I don’t know what to do if I change hers it cost money and if I get married again I’d just have to change them again