Not sure what to think

Ok so before I start let me say two years ago I caught my SO sexting his ex. He said he was just messing with her...I let it go. Now fast forward two years I take his phone (since he is using mine for a game for our son) and decide to mess with his friends before finding somewhere to eat. Mind you, I’ve done this before and they find it funny 😊. Anyways, I found a chat between him and some girl on the PlayStation network. (The basic sexting stuff “more pics” and what not). The only pic she was clothed but talks about being horny, not having a site to cum, the cam not allowing certain things, and so one really got me upset. When I comforted him about it he got angry saying he didn’t know what I was talking about (he did the same thing the first time). When I showed him he said he doesn’t know how those messages got there, he didn’t even know they were in there, and I have an over active imagination because I have a hard time believing those type of messages just appear from thin air. Needless to say he got mad. I’ve been trying to act like nothings bothering me but it really is. I have no idea what to say or do honestly. I’ve seen scammer emails before from porn sites and whatnot and am hoping it’s the same type of deal as she messaged first. Sorry for the long post but I just can’t wrap my head around this 😞