debt collection pursued overseas

My son bought a train ticket in Germany on a day trip from a vacation in England several years ago and somehow ended up signing up for some sort of railcard by accident. After returning home he received a bill from a German company for this card (I guess, we could not read the letter because all in German but it was clearly asking for money.) We ignored it thinking it must be a mistake. Now 3 years later we have a just got a huge bill from a debt collection agency in the Netherlands which is the original amount plus a large fee. They are threatening to pursue this through the courts. Can they do this or prevent my son entering Europe in the future? What should we do?