There are two encounters with the ferryman, Vasudeva, and I'm assuming you mean the first.
At the time of the first encounter, Siddhartha has just experienced a type of rebirth. He is filled with exhilaration about his new view of the world but he still has a huge ego that diminishes his value for all but his own perspective. Vasudeva is an enlightened man, but the wisdom he has to impart about the river, Siddhartha is not yet ready to receive.
In chapter 4, Siddhartha states,
"But I, who wanted to read the book of the world and the book of my own being, I have, for the sake of a meaning I had anticipated before I read, scorned the symbols and letters, I called the visible world a deception, called my eyes and my tongue coincidental and worthless forms without substance. No, this is over, I have awakened, I have indeed awakened and have not been born before this very day” (Siddhartha: An Open-Source Text Chapter 4. Awakening).
Shortly afterward, after a night at the ferryman's hut, Siddhartha tells the ferryman that he won't be able to pay. Vasudeva states,
“I did see it,” spoke the ferryman, “and I haven’t
expected any payment from you and no gift which would be the custom for guests to bear. You will give me the gift another time.”
“Do you think so?” asked Siddhartha amusedly.
“Surely. This too, I have learned from the river: everything is coming back! You too, Samana, will come back. Now farewell! Let your friendship be my reward. Commemorate me, when you make offerings to the gods.”
It's ironic, because Siddhartha has just announced his willingness and readiness to be open to the world, but then almost immediately he diminishes Vadudeva's wisdom and writes him off as another one of the "child-like" people.
“all I meet on my path are like Govinda. All are thankful, though they are the ones who would have a right to receive thanks. All are submissive, all would like to be friends, like to obey, think little. Like children are all people” (44 Siddhartha: An Open-Source TextChapter 5. Kamala).
Siddhartha is amused by the ferryman -- still suffering from a preconcieved notion of "true" wisdom and his own superiority.
The second encounter Siddhartha has with Vasudeva is much different. Siddhartha is greatly humbled. Living the life of the wealthy, he developed a self disgust and loathing. His ego was effectively killed when he tried to commit suicide in the ferryman's river.
Greatly humbled after an unsuccessful suicide attempt (he was saved by hearing the word "om"), he sees the river again with renewed interest and the ferryman as a potential guru to teach him how to listen to the teachings of the river.