How does jimmy guess Bud's name in Bud Not Buddy?

Jimmy knew because Lefty Luis wrote a letter saying that he would bring Bud back to him. and when Jimmy read the note when Bud went there he saw him and said I guess you are Bud. A guy named Lefty Luis said he would drop you off so then he knew that Bud was named Bud. That is it if you have any other questions then just write your question unless nobody answered it.