Cancer man problems... please help...

I am a gemini woman that have know a cancer male for about 6 months now. It was love at first sight for me. I didn't really think it was going to turn into anything when we first met, but then he texted me later that night when I got home. He started to ask me to hang out and showed interest in me. He soon asked me to be his girlfriend after 3 weeks. He was very sweet, romantic, thoughtful, and generous he texted me often at the beginning. After we got into one argument after another things seem to get worst. He became possessive and very secretive. He wouldn't open up to me so that made me not trust him. I find him texting with other people, but I'm not sure who they are. He wouldn't answer my calls sometimes and that makes me a bit jealous and wanting to know. I asked him if he had someone else and he said no. it seems like he want to do things his way and wouldn't really listen when we have a conversation. He would be very moody and wanted to dominate the conversation whenever we talked. I got frustrated many times and wanted to call quits, but he would come around and be sweet whenever i say lets end this. He tells me he likes me, but he won't commit. We still see each other every week. He tells me he wants to be friends for now and he'd still like to have sex with me. The sex is great and I've been giving in cause I think that would eventually make him want to be with me. I fell for him and really want this to work out. I've never felt this way for another man before. Please help me...
Do you think it is better to be his friend with benefits or be friends without? Which one should I do for him to commit to me? I'm scared that he just want me for sex and leave me when he find someone more compatible on one hand and scared that he will find sex with someone else if I don't have it with him. Do you think he likes me? What should I do?