My son is four yrs old and his biological father isn't around. The last time he saw my son was when my son was nine months old. The bio father never signed the birth certificate and my son has my maiden name. I am married and wanting to know if I can change his last name to my married name?

First things first, where are you? Just a state is all you have to give (if you are in the U.S. or country if you are not). Laws very by area. Are you wanting to have your husband adopt? If so the first thing you do is find an adoption attorney in your area. Does the bio dad pay support or has he been acknowledged as the legal father at all? I ask because if he is willing to allow the adoption it can all go very smoothly.

A birth certificate is a legal document. You have to go to court to change it to your husband's name. The court will probably require that the bio father be identified and his input solicited. Or, at least an attempt to.

I strongly suggest you hire a local attorney who can lead you through the process.

OP doesn't need to change her son's BC to change his last name. Simply apply to the court for a name change. As stated, the requirements for a name change would depend upon which state or country OP and her son are in.