My wife of 17 years had an affair last year and is pregnant. The boyfriend knows all about the pregnancy and is demanding that the baby have his last name even though my wife is still legally married.
Now, I don't need to hear about leaving her. We have been separated for over 10 months and have been seeking counseling during that time. We are working on our marriage to determine if there is a future for us.
She is the one not wanting his last name attached to this little girl. This is occurring in South Carolina.

He can petition in court for it, it will be up to family court if the two people can not agree.

You need to get a good family court attorney

If he is the father, he will get visitation rights, some custody rights and the such, so you may as well try and work things out, you will be seeing him for the next 20 years

A peternity test is needed. But he has every right to exercise his parental rights. I don't think he can force that the birth certificate have his last name, but he can force a hyphenated named.