If he is not in to you anymore, and a girl has a natural intuition about her best friends loyalty to her. And if u felt that intuition, then he is definitely cheating on you. So dump him. Go straight in to his face a say that you know that he was cheating on you and say it is over between you guys. Make him cry. Make him wont you back.
If you are the nice kind of person and hate to make people sad, go over and say to him that you know he was cheating on you and tell him that it is over, but in a sad way. Then just walk way and make a sniff sniff sound. Almost like you were crying and hang your head down.
Do not worry there is a special someone at there right now just for you. Just keep waiting. You will find him one day.