No, don't tell him. You are allowed a little privacy about where your thoughts wander, and if he is no more mature than you are, it might blow up into something that is not in fact worth it to fight over.
Incidentally, if you really are 13, I doubt you should be anywhere near websites like that. You never know where you will leave a trail, on the Internet.
Yeah, you should totally tell him. You may be surprised at how glad he is that you told him. It is a good thing that you want to share with him what happened.
Would you be mad if he did the same? It sounds like an accident. I personally don't think porn is cheating. And live cam's is borderline. To me, cheating is actually touching another person. But we all can be curious. I think you can talk to him about it if you want to but not in the way that questions if you were cheating or not. I'd make it a funny story. But then he might tell you one of his 'funny' stories and you have a whole new thing to discuss in your relationship. Which isn't a bad thing. :>)