I am always very curious, so if I have a child, I really want to know.
Especially when he uses MSN. You never know what is in his mind. If he is acting weird to you, reading his conversation can be a way to know why he is acting strange if he won't answer your question.
The thing is thrust will go down and he will think that he is being treated like a little child.
Just look out what he is doing. If you really don't want to read his conversation, you should talk about it with him. Even if he is thirteen, you are responsible for him. You should talk about the consequences about what can happen if he talks for example with strangers or downloading stuff.
He deserve his privacy after all.