If he cheated on the 5th day, and that was the only time, I would cut him some slack. On the 5th day of a relationship, you could hardly expect him to be totally devoted to you. Over time, he could develop this devotion and turn into a faithful man.
If he is otherwise an attentive bf and treats you well, then you might forgive him. If he does not treat you well, then dump him, and dump him if he ever cheats on you again.