My boyfriend and his ex

Okay. So I know my boy friend would never cheat on me.... but he is always with his ex. I go to the middle school close to his high school ( I’m in 8th grade and he’s in 9th) but he is always with her. And I get it because they are friends but he goes over to her house all the time. And they sleepover at his cousins house and she always says that she will get him back. What should I do?