He is using you because he knows that you are an easy target, because he knows that you'll take him back everytime. I was in a relationship about 3 or 4 years ago when I was younger. I constantly let the guy back in, and one day it dawned on me, "Wait a second, what the am I doing to myself?" It sucked, the constant up and down emotional roller coaster ride. You don't deserve that at all. I thought that I wasn't going to find another relationship and about a year later, I met the most amazing guy for me, and we'll be living together next year until we establish ourselves enough to get married.
Bottom Line: Don't let him take advantage of you. You don't deserve it, and its up to you to not let him step all over you.