What to do...

My boyfriend and I recently took a week break. We live together and I stayed at a friend's house during the break. He asked me to come back home on Friday we were fine until he got trashed with his friends on Sunday and one of his friends texted me as him saying he was done and I should go back to my friends that night. He didn't come home til 6:30 the next morning. He said he didn't know anything about the texts and I do believe him that he didn't send them because I already thought it didn't sound like him but I'm sure he was there. He was trashed so he may not even remember. He doesn't remember half the night. We talked and he said he wants to work things out and give us another try. Well I knew he had been snapchatting this girl he met a few weeks ago and it's one ofnhis friends cousin. I told him I wasn't comfortable with it but he still Snapchat her. Then tonight I asked to see his phone and he got defensive and wouldn't let me. Usually he has no problem with me around his phone. I ended up finding messages from this girl on Monday the 3rd.