It's a tough one. I was expecting worse though, though this is pretty bad, too.
Several things can happen if you tell him. One he will forgive you and think that if you are so honest with him about something like this, then he can trust you for bigger things and this might actually make him trust you more.
On the other hand, it could do the opposite. He could start mistrusting you from that point on. It's really a tough decision.
People don't realize that when you do things like that, it cheapens the kiss for your loved one. For your relationship to be special you have to do things that only you do with your loved one. When you kiss, don't give away your kisses to just anyone, or they won't be special, anymore. For something to be real and lasting it has to have a meaning, it has to be dignified, pure, honest. What is a person without principles. Your actions define you, and they can destroy you, or make you beautiful.