How do i get my husband to understand that our relationship is OVER?

I left him 10 months ago. He cheated and had/has a drug problem. He stole from me and our son and was abusive. I still speak to him when he calls but i have told him i don't love him and don't want him back, ever. i would like for us to be civil to each other for our sons sake and i still care about what happens to him, but am i keeping his hope alive by just talking to him? He is a Leo and headstrong and full of himself and used to getting his way, so he thinks eventually when i see positive changes in his life i will change my mind. I need for him to understand that we are over. Will it take divorce papers for him to get it? I can't afford a divorce yet but i am working on it. Any ideas on how to make him understand that it is over?