I assume that you are asking about what happens in Chapter 7 when Bernard and Lenina first meet John the Savage. Bernard questions him about how he came to live in the pueblo and becomes excited about his answers. The reason for this is that Bernard is thinking that he can use John for his own gain back in England.
Bernard has been having trouble with his boss, the Director. The Director wants to exile Bernard to some island because Bernard doesn't really act the same way as everyone else. When Bernard meets John and hears his story, he knows he can use John against the Director. Because of John's story and things the Director has told him, Bernard has realized that the Director is John's father. In the World State, actual biological parenthood is considered disgusting. Because of this, if Bernard can bring John back to England, he can discredit the Director. Once the Director gets fired, Bernard will be safe. That is why he is so excited to hear the answers -- the answers make clear that the Director is John's father.