Aztec Revenge (Aztec #6) by Gary Jennings, Junius Podrug

Juan is a "mestizo," of mixed Spanish and Indio heritage, sure to be doomed to a short, violent life begging on the streets. But Juan has a special gift for handling horses and an ability to defend himself at a time when a man's best friends are his horse and his sword. Only El Mestizo, the half-caste son of the conqueror Cortés, knows that the blood of kings flows in the young beggar.