The Prince of Egypt

In Ancient Egypt, Yocheved, a Hebrew mother, and her two children, Miriam and Aaron, watch as the newborn Hebrew boys are taken and ruthlessly killed as ordered by Pharaoh Seti, who fears that an alarming increase in Hebrew slaves could lead to rebellion. Fearing for her own newborn son's safety, Yocheved places him in a basket afloat on the Nile River, not before bidding him farewell with a final but powerful lullaby. Miriam follows the basket to the Pharaoh's palace and witnesses her baby brother safely adopted by Queen Tuya, who names him Moses.
Years later, Moses and his brother Rameses are scolded by their father for accidentally destroying a temple during one of their youthful misadventures, though Moses tries to take the blame. That evening at a palace banquet, Seti, who wants to give Rameses the opportunity to prove that he is responsible, names him Prince Regent and gives him authority over Egypt's temples. As a tribute, the high priests Hotep and Huy offer him a beautiful young Midianite woman, Tzipporah, and Rameses gives her to Moses. Rameses then appoints him Royal Chief Architect.
Later that evening, Moses follows Tzipporah as she escapes from the palace and runs into his siblings Miriam and Aaron. Miriam is overjoyed to see her younger brother again, but Aaron is fearful to watch the confrontation. Despite Aaron's attempts to protect his sister, Miriam tries to tell Moses about his past, but he refuses to listen. Miriam then sings her mother's lullaby, which causes Moses to remember the melody. Moses runs to the palace, eager to return to familiar surroundings. The truth about his past is later confirmed by a nightmare, and finally by Seti himself. The next morning, Moses accidentally pushes an Egyptian guard off the scaffolding of the temple when trying to stop him from whipping a Hebrew slave, and the guard falls to his death.
Ashamed and confused, Moses flees into the desert in exile, despite Rameses' pleas to stay. While in the desert Moses defends three young girls from bandits, only to find out their older sister is Tzipporah, whom he helped escape from Egypt. Moses is welcomed by Tzipporah's father and the high priest of Midian, Jethro. After assimilating this new culture, Moses becomes a shepherd and marries Tzipporah. One day, while chasing a stray lamb, Moses discovers a burning bush through which God tells him to go back to Egypt and guide the Hebrew slaves to freedom. God bestows Moses' shepherding staff with his power and promises that he will tell Moses what to say. Moses and Tzipporah return to Egypt, where Moses is happily greeted by Rameses, who is now Pharaoh.
When Moses requests the Hebrews' release and changes his staff into an Egyptian cobra, to demonstrate his alliance with God, Hotep and Huy boastfully recreate this transformation, only to have their snakes eaten by Moses' snake. Far from being persuaded, Rameses doubles the Hebrews' workload. Moses inflicts nine of the Plagues of Egypt, but Rameses refuses to relent despite each plague being worse than the one before. Against Moses' warning (foreshadowing the final plague), Rameses makes it clear that he will never release the Hebrew slaves. Disheartened by Rameses' words, Moses prepares the Hebrews for the tenth and final plague, instructing them to sacrifice a lamb and mark the doorposts with the lamb's blood. That night, the final plague kills all the firstborn children of Egypt, including Rameses' son, while sparing those of the Hebrews. The next day, a grief-stricken Rameses, mourning the loss of his son, finally gives Moses permission to free the Hebrews. Moses breaks down crying from the guilt of hurting Rameses, and from hearing the cries of numerous families across the city.
The following morning, the Hebrews leave Egypt, led by Moses, Miriam, Aaron, and Tzipporah. At the Red Sea, they discover that Rameses is closely pursuing them with his army. Upon the arrival, Moses uses his staff to part the sea, while a fire blocks the army's way. The Hebrews cross the open sea bottom; and when the fire vanishes and the army gives chase, the water closes over the Egyptian soldiers, sparing Rameses alone, who is washed ashore on the other side of the sea. Thereafter, Moses sadly bids farewell to his brother, and leads the Hebrews to Mount Sinai, where he receives the Ten Commandments.

Question: Who does a grief-stricken Rameses mourn the loss of?
Answer: His son