Please help, trust issue in relationship.

My bf and I have been together for 13 years. He’s always told me he loves me and he wants to get married.

He has a cell phone that he uses for work purposes. About a month ago, he left his phone unlocked and open to his text messages. I saw a message from a girl named Mel who was asking him to call her because she needed a ride somewhere. He called her cutie but told her he was at work.

I know I shouldn’t have read the message but I don’t hide anything from him, my phone is unlocked and on ring. He has never done anything to make me doubt him until now. Should I confront him about the message or just be patient and let it play out?