Would going to court for abandonment be a wise option?

My ex and I have 2 children. We are no longer together and have never been married. He lives a few hours from where the children and I are living. He has not sent consistent child support since we have split up. The last time was probably a year ago and the time before that was about a year ago. He has not physically seen the children in about a year and a half. He rarely calls them as of today the last time he spoke with them was July. He normally does this were he will call for a month or so consistently and then will disappear for several month. He does not try to contact them on special holidays of any kind. At this point he is a stranger to them known only by old memories and a name. My current fiancé has raised them and supports them and would like to adopt them. We are concerned that we will go to court and lose and he will continue to hurt them.