trust issues???

so i've caught my boyfriend a few times talking to other girls, sexually. the last time was this past may. but of course he said it was a huge mistake, and i want to think he learned his lesson because i almost kicked him out. Now i've noticed he takes his phone EVERYWHERE. I went through it a few times when he was sleeping, and i noticed he was talking to this girl, nothing bad just casual, but then he deleted his messages with her. Am i being paranoid or is he just deleting them because he is paranoid i'd assume the wrong thing??? I need to talk to him about it but idk how to bring it up. Help!!!