I want to change my sons last name and my fiance wants to adopt him

Please do not attempt to gather meaningful insight into this issue through brief questions on a message board.  Consult with an attorney who has experience in Family Court and adoptions.  Every adoption is different.  Even what appears to be a no-contest agreement in the realm of adoptions can have major pitfalls.  Through 18 years of practice and countless adoption cases each one is different.