Orestes, who was partly of Germanic blood, was historically the magister militum — the senior officer of the Roman army second to the emperor. He had indeed promised his Germanic foederati a third of Italy to settle in but not to Odoacer personally. Orestes was himself a usurper, having promised the land to foederati in order to buy their quiescence in his rebellion against the legitimate emperor Julius Nepos. Although Orestes proclaimed his son, Romulus Augustus (derisively called Augustulus, meaning "little Augustus") Emperor, the proclamation was not recognized as legitimate beyond the parts of the Italian Peninsula controlled by Orestes' troops. The common identification of Augustus as the "last Western Emperor" dates only from the publication of British historian Edward Gibbon's History of the Decline and Fall of the Roman Empire in the late 18th Century.
At the time of his proclamation as Emperor, Romulus Augustus was 16, older than portrayed in the film. Upon being captured by Odoacer, the boy usurper was seen to be a victim of his father. He was spared and sent to live in comfortable retirement in southern Italy.
There was a struggle for control of the Eastern Roman Empire at the time but the Emperor alluded to would probably have been Zeno (explicit in the novel). In truth, Zeno never recognized Romulus Augustus but recognized Julius Nepos as Emperor in the West until the latter's death in 480. At the behest of the Roman Senate, Zeno recognized Odoacer's administration of Italy; both he and Odoacer himself maintained the pretense of Nepos' reign. The Eastern Emperor eventually sent the Ostrogothic foederati under Theodoric the Great to depose Odoacer when the latter proved disloyal.