Moses warned Pharaoh about the killing of his firstborn son in Exodus 11:
4 So Moses said, “This is what the Lord says: ‘About midnight I will go throughout Egypt. 5Every firstborn son in Egypt will die, from the firstborn son of Pharaoh, who sits on the throne, to the firstborn son of the female slave, who is at her hand mill, and all the firstborn of the cattle as well. 6There will be loud wailing throughout Egypt—worse than there has ever been or ever will be again. 7But among the Israelites not a dog will bark at any person or animal.’ Then you will know that the Lord makes a distinction between Egypt and Israel. 8All these officials of yours will come to me, bowing down before me and saying, ‘Go, you and all the people who follow you!’ After that I will leave.” Then Moses, hot with anger, left Pharaoh.
Did Pharaoh know about the blood that could have saved his son mentioned in the next chapter?
12 “On that same night I will pass through Egypt and strike down every firstborn of both people and animals, and I will bring judgment on all the gods of Egypt. I am the Lord. 13 The blood will be a sign for you on the houses where you are, and when I see the blood, I will pass over you. No destructive plague will touch you when I strike Egypt.