cheating

My boyfriend and I have been together for 12 months and will be a year in January. I recently jus found out that he kissed someone else a few months back but he swears he doesn’t care about her and she initiated the kiss... I love him and I don’t want us to break up but how do I know he wont ever do something like this again or something even worse? I wanna go through his phone because he never lets me even tho I honestly have never felt the need to till now. He says hes sorry and he loves me and that he wont talk to her but I really dont know what to do. Even though it was a kiss it still hurts that he kissed someone else..