Did Sha'ul not know David and his father? They told him "Raisi Ben l'Yishai", and "vaYe'ehave'u Me'od va'Yhi Lo Nosei Kelim" (16:18, 21)!
Rashi citing Yevamos 76b: He saw David conducting with royal etiquette. He said, if he comes from Peretz, he will be king, for 'a king may be Poretz (breach) a wall to make a path for himself, and we do not protest. If David comes from Zerach, he is merely important. 1
Radak citing his father: When Sha'ul saw David go to fight Galyus without fear, he asked if his father was a strong warrior, that we may rely on his son. Avner did not know, for Yishai was old (he had not been in the army for a long time).
Malbim #1: When Divine light appears on a spiritual person, he is transformed to a new person, and his face radiates. Therefore, Sha'ul did not recognize him now.
Malbim #2: Sha'ul had promised his daughter to the one who will kill Galyus. He needed to fulfill this, so he asked about the lineage and standing of David's father, if he is proper to marry into the king's family.
If he knew only that he is from Yehudah, he should have discussed also if he comes from Shelah! Radak says that Zerach is like Ezrach, but Rashi and the Gemara did not expound the name Zerach! Perhaps Sha'ul meant that also Bnei Zerach are important, for it says "va'Tahar Lo" (Tamar became pregnant with sons like Yehudah - Rashi Bereishis 38:18), but if he is from Shelah, there is no guarantee even of importance. (PF)
