Your master’s son - Meaning Saul’s grandson (2 Samuel 9:6). David asks the question, evidently hurt by the apparent ingratitude …

And the king said, and where [is] thy master's son ?
&c.] The son of Saul, who was Ziba's master, meaning Mephiboshe…

Ziba slandered Mephibosheth. Great men should always be wary of flatterers, and be careful to hear both sides.