K.L.
I agree with Karee W but I'd like to point out one thing that nobody else has said. I just want to make clear w/o a doubt that this behavior is damaging to the younger child. The older child needs to learn (if he doesn't already understand) that doing such a thing is extremely disrespectful and degrading to the other person. This goes way beyond "boys will be boys". You sound like you might know that already but it's really important to stress this to the child. Good luck!