My question involves a child custody case from the State of: Texas

My fiance has just recently finalized his divorce due to the fact that he could not afford an attorney and she was unwilling to sign the papers he needed to file himself. Regardles, it is now finalized and she has yet to contact him about their son. The child lives with us and my fiance is the managing conservator. She does not call and has yet to try and see the child since the divorce. In fact I have never met the woman because she has not seen the child for as long as I have known him. He only knows me as his mother.
My question is, is there anyway to just revoke her rights? Can we get her for abandonment? If so how long would we have to wait to be able to file for abandonment? Or is there anything else we can do? Just wondering what our rights are. This is not fair to the child. I don't see how it will be ok for her to show up one day and want to take him when he has no idea who she is. I feel that it may scare him. He is just a toddler, will be 3 in October.