Hanging Out with Ex

So my ex and I were together on and off for almost 4 years. We fought a lot. We broke up in February and I didnt talk to him for three months despite his effort to reach out to me. Now we back talking. Previously when we were together I was upset that he spent more time with his friends then me. Now hes been spending time with me, taking me on dates, he came clean about some things he did when we were together, Ive been back around his family and hes been being really sweet. He tells me he just wants to be friends and see where things go. We also havent been fighting.

BUT....
He has a girlfriend. Who he denies. But I know he lives with her. Me and him both were in college living in dorms but we decided to move back home. When I go to his house he has no clothes or personal hygiwritingene items there. She posted a picture on twitter last month of her writing in her diary on her ipad that passage she was writing was talking about her "wonderful boyfriend". But she doesn't tweet stuff like "hanging out with my baby" or take pictures of him and her hanging out together and post them on twitter and facebook like most girls do. I THINK shes also paying for his cellphone (when he calls my house he name pops up on the caller id) and car insurance. She is also best friends with his best friends girlfriend.

When I confronted him he said he's not in a committed relationship with anyone. He said it wouldn't be fair for me to have to wait on him until hes ready to be comitted and that he doesnt care if I date other guys. He says he loves me and care about me and want me in his life and that he wants to work on our friendship and see if we can turn it into a relationship.

So should I back off? Or just be "friends" and continue to hang out with him? Do you girls think he's using this girl? Or that their relationship is out of connivence since the whole bestfirend thing? I dont know what to do...