Most Helpful Guy
Most relationships start with sex outside of the relationship. I doubt if he suddenly told you "Let's be bf/gf" you'd just sleep with him, so he's lost interest.
Imagine if you tried to kiss a guy and he said "No, I'm sorry". Women don't take how rejection affects men into account because since they never have to make the first move, they never actually experience rejection.
Pretty much this is how your relationship currently is; he wants sex, you want a relationship. You won't give sex without a relationship and he won't give a relationship without sex. Unless one of you changes your mind, he'll just move on.