It's ok to get rid of people in your life that have a negative impact on you. It sounds like you have given him several chances because you liked being around him as a friend. You could give him another chance if you wanted or you could say enough. I understand that you feel sorry for him but he is not your responsibility so I don't think you should feel like you have to do anything for him at all.

In that case, what I would do is tell him that I no longer want to see him or communicate with him, let him respond and then tell him I'm done. After that I would simply avoid him in hopes that he gets used to not having me in his life and eventually moves on. But that's just my opinion based on the limited information I know. Regardless of what you do, I'm sorry you went through that. People are here to support you.