My question involves name change laws in the State of: Texas

Okay, here is my question. I'm a 27 year old mother to a almost 6 year old son. I am engaged to be married in the spring of this year. My sons biological father is on his birth certificate, we were never married but were beyond certain he was the father, as I don't sleep around. My ex and i split up almost 4 years ago to the day and he hasn't seen our son in a little under 4 years, he saw him for about two months after we split up. Then he hightailed it out of the state and left
no forwarding number or address, and instructed his sister, my brothers wife not to tell me anything. He moved back into Texas a year ago I think? He has contacted me once since he moved back and said he would only be a part of my sons life if him and I could be together and when I said I wad happy and didn't want to be with him but he could see his son but we needed to do it right through the courts he said no and didn't want to see him. I only said through the courts because when we split he threatened my life and threatened to leave the state with our son... Basically, my son doesn't know who his real father is but considers my fiancee his dad, as he raises him as his own son. With my ex having a bad wrap, would it be hard to change his name to mine or revoke his rights? It's on file with CPS in Texas that he abandoned him and was on drugs. He filed complaint on me with CPS and left the next day and according to caseworker he wasnt mentally stable. All charges against me were dropped BTW, concluded they were filed out of spite. Any advice would be helpful!