Why is my ex keeping me around?

I am going to be a senior in college. I dated a guy for 3 months. I liked him a lot and he liked me a lot. However, he didn't want to work on the relationship and felt like he was too immature so he dumped me. We were in the same classes for 3 months after that and I know for sure he still liked me and was hurt. When we went on summer break, things changed. He stopped talking to me and took his space and now we are best friends. I see hum more than any of my friends at school and he sees me the same amount. We do work together but on friday and saturday nights we usually hang out with our separate groups of friends. We are in the same major but our close friends are in different ones. He has told me twice already that he only considers me a friend and doesn't want to mess up our friendship. He hasn't dated anyone else since and I haven't either. I haven't because I haven't met anyone who could replace him. Why does he keep me around? Why does he do homework with me? He was immature but if he has no intention of dating me in the future why is he keeping me around?