The answer to this question can be found in Chapter 11, but you also need to be aware of what happens at the end of Chapter 10, which explains John's sudden popularity and why everyone wants to see him so much. Chapter 11 starts by saying:
After the scene in the Fertilising Room, all upper-caste London was wild to see this delicious creature who had fallen on his knees before the Director of Hatcheries and Conditioning - or rather the ex-Director, for the poor man had resigned immediately afterwards and never set foot inside the Centre again - had flopped down and called him (the joke was almost too good to be true!) "my father."
It is at the end of Chapter 10 when this incident occurs as Bernard Marx presents John and Linda to stop himself being sent to Iceland as a punishment. The scandal that is created by John being introduced to his father and the subsequent retirement of the Director makes him the gossip of the upper-class castes and therefore instantly popular.