InChapter 13 of John Boyne's The Boy in the Striped Pajamas, offer a reasonable explanation for the lack of communication between Lieutenant Kotler an his father.
In John Boyne’s novel The Boy in the Striped Pajamas, Lieutenant Kotler is a 19-year-old officer in the German Army, specifically, in the SS, who enjoys hanging around Bruno’s house and interacting with the young boy’s family. Bruno’s father, of course, is the commandant of the concentration camp at Auschwitz, a position for which he was appointed by Hitler himself. In Chapter Thirteen, Kotler is having dinner in the commandant’s home along with the latter’s wife, Bruno and Bruno’s older sister, Gretel. During dinner, the conversation turns to the subject of Kotler’s father, who the young officer has noted left Germany for Switzerland in 1938. Hearing this, the commandant becomes curious, and suspicious, that any ‘good’ German would think to leave the Fatherland at such an important moment in history:
“And what reason did he give, I might ask,” continued Father, “for leaving Germany at the moment of her greatest glory and her most vital need . . .”
Lieutenant Kotler professes ignorance of the reason for his father’s abandonment of the family and departure for Switzerland. Pressing the matter, the commandant suggests, perhaps, that Kotler’s father had “disagreements” with the German government, implying, of course, that the father was disloyal to the Fatherland:
“One hears tales of this from time to time. Curious fellows, I imagine. Traitors, others. Cowards, too. Of course, you have informed your superiors of your father’s views, Lieutenant Kotler?”
To this, Kotler has no answer. The reader can, however, surmise that his father did, indeed, have disagreements with the Nazi regime ruling Germany, which had annexed a very compliant, willing Austria during 1938, as well as occupying the Sudetenland region of Czechoslovakia. Germany was on the path to war, with British appeasement and French ineptitude providing an inviting path toward domination of Europe. Kotler’s father may very well have been afraid, or seriously opposed to such policies. There is, however, a more likely explanation: Kotler’s father was Jewish, and fled Germany to save his life. Nineteen-thirty-eight was also the year of Kristallnacht, the “night of broken glass,” a major attack on Jewish families and businesses and the beginning of the more serious repression of Germany’s Jews. Additionally, earlier in the novel, it is suggested that, possibly, Kotler dyes his hair, and that is actually darker than Kotler lets on, a suggestion that he not the ideal Aryan figure to which he pretends. In short, the reader can logically conclude that Lieutenant Kotler is Jewish, and hasn’t communicated with his father because the latter fled the onset of the Holocaust.
Certainly, Bruno's father has drawn the conclusion that his loyal subordinate is Jewish, as he ultimately has the young officer sent away, presumably to his death.