My boyfriend cheated on me but I'm afraid he's still doing it?

My boyfriend and I are both seventeen. Earlier this month, my boyfriend of six months broke up with me and wouldn't tell me why. The night he broke up with me, he came over and said he wanted me back but he had to tell me why he dumped me. He told me that around the time we started dating, other girls that he is friends with he had liked before me started liking him. He told me that for a while he had flirted with one of them and other times when he told me he was at work he was at another girl's house. He told me all the did was kissed and he kind of touched her and he has sexted her before. He was very distraught and sincere sounding. He cried and said that I didn't have to take him back. I love him, so therefore I did. He told me he would never talk to the one he kissed again. I believe him but now, like two weeks later he has started to be secretive with his phone again (like he always had been before). I don't want to make assumptions but he gets mad whenever I try to touch it. One time he has let me see it and her name wasn't in there, but he always gets texts from this number that isn't saved and I'm very scared that it's her..I don't want to be nosy but I feel like I should be able to know but I don't want him to get mad at me :/ Do you think he could still be cheating on me..? Can someone please answer? :/