I don't understand .. (Long)

My boyfriend and I have been together for a year and almost three months. He is currently staying at my house because he's from Minnesota and I live in Colorado. An old friend (male) of mine texted me. We haven't talked in about three years. My boyfriend automatically flipped out and thought I was cheating on him because of previous relationship problems. I'm not cheating. Yes I did at the beginning of our relationship and we said we would work it out. When my friend texted me my boyfriend thought it'd be appropriate to text him acting like it was me JUST to see if anything suspicious went on. Nothing did and now that I won't let him go through my phone (he wants to look at all my deleted texts and things) he is leaving. My mom is also pissed that he's trying to go through my phone because it's technically her phone because she pays for it. UGH