If you truly love this guy, then you both need to come clean. However, I feel the need to say that I don’t think that you do truly love this guy. If you have cheated on him “a couple of times”, it seems apparent that you are just looking for someone to be there to give you what you want. That’s not a bad thing. That goes back to what I said about you being young.
In life, you are going to get hurt, and you are going to hurt others. Whether or not he is cheating with this other girl, or whether or not something happened between them on the day of question, I think, is not what you need to be questioning here. I think that what you need to be questioning is what you really want out of him. If he does not make you happy, or he does not fulfill your needs enough for you to cheat on him, then you need to find someone else.