What does 1 Samuel 17:55 mean?

When Saul saw David heading out to fight the Philistine, he asked his general Abner, 'Whose son is that young man?' Abner replied, 'Honestly, I don't know, Your Majesty.' 1 Samuel 17:55 - Modern Text Bible

(Saul is curious about David's family, probably because of the promise to reward the champion's family. Abner, his top officer, doesn't know either.)