Are cheaters always cheaters?

My bf is with me for a year and half and we’re talking of settling down i’m scared because he cheated on his ex so many times (not sex) but kissed other girls and did stuff behind her back and she left him. We started dating and he told me she would cheat and do stuff wrong and so he did it. He never had her on social media and has me all over his social media for over s year now. He tells me he never felt this way about someone and whenever i mention what he did to his ex he gets upset because it’s in the back of my head. can people change? He was with her for 3 years and lived with her own apartment and all and that’s what scares me. Although it is confirmed that she did cheat on him first as she even told me way back. Advice? He says he doesn’t even want to talk to other girls because i’m all he needs but i’m very scared. Help?