Is he using me for sex?

I have been dating this guy for a total of 6 months. We dated for four months, ended it. And are now back together. The first time we were talking I kind of knew he only wanted sex because he told me he wasn't ready for a committed relationship. So I had the talk with him that I was more than friends with benefits. I guess it scared him off. About two months later he texted me telling me he made a mistake about saying he wants to see other people and that he wants to put effort into me. He takes me out to dinner once a week, bought be Christmas presents and our sex is really good.
The only thing I'm concerned about is if he came back because the sex is good or if he actually likes me because of the effort he puts in. I want to be able to just enjoy what we have now, but its on the back of my mind constantly and is always nagging me. Should I ask him if we are exclusive? Or does it seem like maybe he just came back for the sex?
Is he using me for sex?
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