I found a text message im my boyfriends phone from another girl. Is he cheating?

In the text message he was asking her for and she said no. Even though she said no he could have done it with someone else i don't know about. I confronted him about it and he said he made a mistake he sorry boo hoo he will never do it again. he said he didn't cheat but i dint believe him. We have been going out for a year.