"Vasudeva said: 'Take [your son] into the town; take him to his mother's house. There will still be servants there;...if they are no longer there, take him to a teacher, not just for the sake of education, but so that he can meet other boys and girls and be in the world to which he belongs. Have you never thought about it?' 'You can see into my heart,' said Siddhartha sadly. 'I have often thought about it. But how will he, who is so hard-hearted go in this world? Will he not consider himself superior, will he no lose himself in pleasure and power, will he not repeat all his father's mistakes, will he not perhaps be quiet lost in Samsara?' The ferryman smiled again. He touched Siddhartha's arm gent;y and said "...Do you then really think that you have committed your follies in order to spare your son? Can you then protect your son from Samsara? How? Through instruction, through prayers, through exhortation? My dear friend, have you forgotten that...story about Siddhartha, the Brahmin's son, which you once told me here?Who protected Siddhartha the Samana from Samsara...? Which father, which teacher, could prevent him from living his own life, from soiling himself with life...from finding his own path...if you were to die ten times for him, you would not alter his destiny in the slightest'" (120-121)
Siddhartha's "reverse culture shock", or the rough patch of his return, occurs when he understands he cannot change his sons fate. He sees himself in his young son, and is what is so frustrating to him. Siddhartha would like to shield and protect his son from all the things that once plagued him. However, while speaking with Vasudeva, he understands no matter what he does for his son, he could never change his son's destiny. The ferryman makes very good points to Siddhartha. You must go through all the trial and error in the world, to find your true self, and meaning. If his father tries to block that part of his son's life, it takes away potential wisdom. This is true, because wisdom comes from only experience. His father could hold him back, and tell him that there are terrible and toxic things in the material world. This would not teach him anything though, he would need to learn from his experiences. The fact it is so hard for Siddhartha to decide what he is going to do with his son, is the reason this is "crossing the return threshold" for him.