X- wants to change my sons name

I was with a girl off and on for about 9 yrs. we had a son named him after me. About the time my son turned 8 my x-gf and I broke up. Before we broke up she says that she was unfaithful and that my son is not biolodgicaly mine, I wouldnt doubt she would say that out of spite or to hurt me he looks nothing like me AT ALL and I guess I always had a feeling deep down but never felt the need to ask such a ? anyway its two years later my son is ten I see him when I can which is basicaly when she allows , on ave.once a week. I dont pay child support because long ago she signed a paper waiving any support from me. I do have him covered on my medical ins. and also a college fund in a bank account opened by his mom but I have deductions from my chk weekly going into it.Recently his mom married and says she wishes to change his last name to what I believe she meant was her old last name regaurdless of what I say. My ? is can this be done? Without my consent? If he really isnt mine and ordered to take a paternity test can she do then? My name is on the birth cert.F.Y.I. her reasoning behind this was because she has raised him the last ten years and I havent so she deserves all the credit not me. I couldnt believe this, can you?