How to terminate my ex's parental rights

[Vinsant5@aol.co]

What is the name of your state? FL
I am divorced and remarried. My ex and I have two children, one which he has no rights to because we weren't married when she was born and when she was two he signed away his rights. We married when she was 7 and had alittle boy. He was abusive, physically, mentally, and sexually while we where married and the children remember it well. He never hurt them though. He has never paid child support for our daughter, she's mentally handicapped and recently diagnosed with bipolar and very delusional. He has never been there for her. He has only been paying child support for our son for the last 15 months. I have to remind him every month and it's never what he says it will be. We only live three ours away, yet he only gets the kids about 3 times a year. He calls about twice a month to talk to our son. He is not involved in their lives in anyway and my son has come to me and asked for his step father to adopt him. Mike, my husband, coaches his baseball team, goes to school events and conferences for both children, and is always there for my daughter. We have explained to our son exactly what adoption intels and he says that it is what he wants. My ex on the other hand said he will never let Mike adopt Devan. How can I have his rights terminated or is it possible?
Stressed out in The Sunshine State