You can't just know that he is. You have to be absolutely sure. Have you seen your boyfriend cheat on you? Have he became less interested in you? Have there always been an excuse for him to not talk to you?
Obviously the best idea is to break up with him. But we don't know if he is really cheating on u. Have u seen him cheating on u? Kissing another girl? Not talking to you? Not hanging out with you? anything like that. And if those are yes's then he is cheating. Also u need to talk about it sslllloooowwwwlllllyyyy:) Don't rush it. then when he already gave up. that's when you should say "its over"
If you know for sure and you are asking advice from strangers then you must have some thought of what you want to do. You can keep dating him and see other people. He's probably not going to like that but you don't like what he did either. You can forgive him and keep seeing him. Or you can break up with him. Not knowing him or you I can't really advise you.