NameChange for my son

California-My 10-year-old son lives with me and my husband of 5 years. I have full physical and legal custody of my son. We have been together 8 years as a family. My son's father wasn't around so my son's last name took on my husband's name. He is registered in school and his medical records under my husband’s name. I would like to have my son's last name legally changed. I have copies of my divorce papers showing full custody. All forms of ID that I have for my son are under my husband's last name. My son's father is in the picture now and has every other weekend to visit with my son and pays a small amount of child support. I briefly mentioned this to my ex-husband and he disagrees with me on the name change. Please advise as to how I would need to go about getting his name changed and any links to free legal forms that would need to be completed. Is there a fee to have your name changed legally or is it possible to go to the social security office with my divorce papers, his school records and have it processed this way. I appreciate your time and effort in responding to my questions. Thanks Again for your guidance. NamChnge