My ex still contacts me but I am trying to move on?

Me and my ex boyfriend have been off and on for 5yrs. The first break up I moved to another state and I thought is was best for us to go our separate ways at the time I was experience allot of stress from the lost of loved one. So, in the beginning of 2010, after a two year break up we came in contact with one another and were living back in the same town we decided to get back together. He has shared with me the feelings he encountered when I first left. His lifestyle also changed completely after our breakup! He is less motivated about his life and priorities. So, our second breakup happened this year of April when I decided to leave him, after he became incarcerated. I have tried to work things out with him even after the fact, not as a couple but as friends. After, moving all of his thing out he still kept the keys to the apartment. He believes he can show up at the apartment we once shared unexpectedly, shower, and eat there as if it's normal! Although we have some of the same friends, When I am with my close friends he shows up there, he invites me to all his family events and he keeps me updated about everything that is occurring in his life. I recently told him I was seeing another man and he acted like a jerk and finally gave me back my keys. Then he went on to say that, he couldn't see him and I together anymore! As if we are not already single people. After this happened he didn't contact me for three weeks. Then just yesterday he contacts me to see how I am doing, what I am doing for the weekend, if I am going to be close by, am I going out with any of our close friends. I am sure he is seeing his ex girlfriend, who he was seeing for about a year during our 1st breakup, an ex who constantly tried to manipulate him into breaking up with me doing our relationship. So why is he contacting me? I am trying to move on with my life. Why do some people think it is okay to remain friends with your ex? I feel the need to isolate myself from him that way I can officially move on. I don't want to argue or have any harsh feelings towards him. I love him yet, I would like to have my own life. Should I completely lose contact with him or just remain friends?