S.G.
Meg, slow down. First of all, he is not a child any more, at 13 he is growing into a young man. Second, we raise our sons to be men, we want them to be strong and stand up for themselves. When they do though, if it is an area we disaprove of, we get upset.
Your young man believes that in your room there must be something of great of importance. Why does he beleive this, because you have proved it to him. If there was nothing to have, in his own mind he does not believe you would be so adament about him staying out. He thinks that you protest too much for there to be nothing of great value in that room.
At 13 he is curious about the life of adults, he is on his way to being one. He is curious about the life between men and women, and that also takes place primarily in the bedroom. He may be looking for things that would help him understand the relationship between men and women a little better.
If the quest a human being is on, is valuable enough, they will continue the quest against all odds. His quest must be very valuable. It must also be something that he finds to be very private because he is not telling you about what it is.
Your room holds a special interest to him.
You are now in such a power struggle that neither of you are going to win but your relationship with him will suffer.
There comes a time in every parents life when we have to realize that we dont know it all and at times we need to step back from the situation and begin again.
Sit down with him and ask him to level with you. Let him know that you are very curious about why he chooses to continue going there. Ask in a curious voice, not an accusing voice. His answer may lead you to a better way of taking care of this.
My guess is, until his curiosity is satisfied this will continue. Is it possible he would like to see what a condom looks like. Or is he looking for other items that would leave him just as curious?