You did that kid a kindness... you are a good man. Hopefully, he will be smart enough to change his ways. If not, it's cop time.
Sadly, my experience says he will ignore your words of wisdom and continue to be an idiot...I would like to be proven wrong.
I have been known to shout "SLOW DOWN, berkeleyER!" at cars that speed on my residential street. The neighbors love me.
Talking to the kid was probably the best choice. For one, he knows somebody in the neighborhood saw what was going on. Second, he may have been shocked to find that he was doing something wrong...Nobody dumber than a teenaged boy. Third, he knows that you know, so he will likely leave your stuff alone.
If he ignores you, next time you talk to him be prepared to box his ears.