[cheebacheeba]

Or, leave the door open, and allow him to come in when YOU give him permission under your terms, you know, make it a trust thing between brothers.
If he comes in at a time other than that which you specify, simply break one item from his room, one item for every time he does this. No exceptions. Do it to something of particular value to the kid the first time around, just to prove you're serious.
Just make sure you threaten him with ample violence if he's thinking about talking to the parents. If necessary, hit him across the ankles with a leather belt.
Hopefully, he won't ever outgrow you.

Don't thank me, glad to be of service.