While I understand the frustration of overdraft fees, let me explain the other side of this story. I work with credit card disputes for a bank, and about 70% of the disputes I handle are fraudulent charges. I find notes on accounts everyday that someone called in pretending to be the cardholder. How much angrier would you have been if some stranger had called in and we had been lax in our security measures?? They could have gotten away with sensitive information that would have certainly led to much more money lost in fraudulent charges to the account.
At the end of the day, if your son is old enough to have a debit card and tour around Europe, then he is a big boy, and should be responsible enough to keep his account balanced and not overspend to these extremes.
On a side note, I will say that most banks will hot-list (place a block on) a card that is overdrawn too deeply or for too long. It does seem strange that they didn't take that proactive measure.