At the time I had my son 5 months ago his father was not in the picture so I opted to not give him his last name or put his name on the birth certificate... however now his father came back and we are working things out.. how do I go about changing the birth certificate and my sons last name... do we have to get a paternity test first?

Check with your local family court. It may requiry that the coiurt order a paternity test, or it may be that he can sign an acknowledgement of paternity and request that the birth certificate be amended.