In chapter 4 page 53
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"But Kino had lost his old world and he must clamber on to a new one. For his dream of the future was real and never to be destroyed, and he had said "I will go," and that made a real thing too. To determine to go and to say it was to be halfway there."
Basically, this means that he had moved toward the fulfillment of a dream, and there was no going back.
Kino's brother is afraid for Kino because he has more knowledge of the world and the way it "used to be" as Kino begins this adventure. He tries to convince Kino to accept the high offer or not go to the city, but Kino will not change his mind.