I think only seeing his parents every other weekend lets him stew in resentment. He might see them just enough to be reminded of his being "ditched" at Tannenberg, but not enough to start to forgive them. Mentioning that they can only spare two days during autumn break because of work, even after sending him to boarding school, probably feels more like a taunt to him than generosity.
Still, he could have worded himself a bit more ... um ... diplomatically? Just look at his mother's face.