I think he's cheating please help

I went through my bf's phone and found him flirting with a coworker. The only reason I was looking through his phone is because he accidentally opened up a snap from her and he got REALLY nervous when I asked him about it, in the text messages he would ask her "where are we going to eat?" And "do you want me to rub lotion on you?" Because she was sun burnt. I need to find out how to get all the messages on my phone. I know his iCloud info...is there anyone that can help?