Just to add food for thought - the relationship between father and son in the book is meant to be reflective of Kafka's relathionship with his real father. Kafka had stated at one point that he felt like an insect in his father's presence. His father was large and imposing in appearance while Kafka was small and weak.
The relationship between the father and the son is strange. It is certainly strained, but it is also complex. The son's metamorphosis begins while works in a job that he hates in order to pay down his family's debt. His father loves him, but his love seems difficult to find: although tears eventually come, his initial response to his son's transformation is not sadness, but anger (he shakes his fist). The twisted part about their relationship is that the son essentially gets sick while carrying his father's burden (debt) but the father, instead of being sympathetic, essentially rejects and attacks the son for becoming the "vermin" that he has become. He hurts his son, physically, and scars him (he throws apples at him which injure his son and eventually cause an infection). This is deeply ironic, because the change in the son seems to occur as a result of the work that he does on his father's behalf. The father should be showering him with love, but instead hates the despicable creature he has become and wants nothing to do with him.
