There is not enough information here.
In general the name change of a child needs both parent's ok. Consult an attorney in person to discuss the specifics of your situation.
This answer DOES NOT establish an attorney-client relationship. This answer is based on the limited information provided and is not intended to be conclusive advice. There are likely other factors that might influence or change the advice after a more lengthy consultation.
Do you know who actual father is? There may be several issues that arise with this situation, it is best if you consult a local attorney over the phone or in person in order to ensure that this is done correctly.