None of the above actions are the best course of action, really.
You are not going to make the scout stop in the long run. All his friends do it too.
To illustrate my point, I will use a real life example involving a teacher and Freshman boys.
Teacher borrows student a's phone on a car trip to use the internet, as he had unlimited internet usage. She and student b are trying to look up the phone # for a hotel they were going to stay at. They saw porn links in the browser history of the student a's phone and even clicked on one (accidentally I assume).
Point is, student a could not argue about the posession and what could the teacher do?
She could punish student a .... but she was the one that asked to borrow his phone in the first place. Plus, that would only make student a hide it even better next time.
In fact, she was even able to get the other Freshman guys to stop needling him about it by stating that "So you've never looked at it before, lets search your phone/computer" and "I don't want him to be embaressed every time he sees me in class again".
The other freshman truly believed that they would be found guilty if their stuff was searched. Nothing more was said about this by anyone probably a day after it happened.
Does he still look? Yes. Does everyone else? Yes. Did the teacher handle it in the most friendly and non-demeaning way? Yes.
One more thing, Kids are smart these days. I have never seen an incoming freshman class before that knew so much about hiding history on the internet.
Finally, don't bother the kid about it. The only thing he will learn is to be more careful around you with it. Not to stop it.