Wondering if he wants me back

First I want to start off by saying sorry for the long story and then getting to the question. My ex and I broke up almost 4 months ago and after the break up my heart was broken. But over the month I have learned to get over the pain and see why we broke up and that it was needed for us. Then about a month ago I truly felt deep down that I wanted to be his friend and let our friendship blossom into something good. But I also realized that I liked him still and wanted to date him later after a good frienship was established. The only thing is that he has a girlfriend now for about 2 and a half months. At first his relationship with her seemed good and it would last a long time. But now his family and others are telling me that she isn’t nice to him and really bossy and controlling and it just makes me think he isn’t happy at all. But he stays with her and is still my friend. Well two days ago was christmas and he called me and we talked a lot and saw each other at the mall…by mistake didn’t even plan it. Well, he seemed really interested in my life still and wanted to know everything. Well my questions are, why does he stay with a girl that hurts him and isn’t nice to him and treats him wrong? Is it the physical part that keeps him there? And the last question the big one, does it seem like he still is interested in me and wants to continue a friendship? And maybe even thinks of me the same way he used to? I just have this feeling that he would call me after he breaks up with her and would start talking to me even more than he is now. Please help me figuring this out.