A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown [1882] at sacred-texts.com
David said, Is there yet any that is left of the house of Saul--On inquiry, Saul's land steward was found, who gave information that there still survived Mephibosheth, a son of Jonathan who was five years old at his father's death, and whom David, then wandering in exile, had never seen. His lameness (Sa2 4:4) had prevented him from taking any part in the public contests of the time. Besides, according to Oriental notions, the younger son of a crowned monarch has a preferable claim to the succession over the son of a mere heir-apparent; and hence his name was never heard of as the rival of his uncle Ish-bosheth. His insignificance had led to his being lost sight of, and it was only through Ziba that David learned of his existence, and the retired life he passed with one of the great families in trans-jordanic Canaan who remained attached to the fallen dynasty. Mephibosheth was invited to court, and a place at the royal table on public days was assigned him, as is still the custom with Eastern monarchs. Saul's family estate, which had fallen to David in right of his wife (Num 27:8), or been forfeited to the crown by Ish-bosheth's rebellion (Sa2 12:8), was provided (Sa2 9:11; also Sa2 19:28), for enabling Mephibosheth to maintain an establishment suitable to his rank, and Ziba appointed steward to manage it, on the condition of receiving one-half of the produce in remuneration for his labor and expense, while the other moiety was to be paid as rent to the owner of the land (Sa2 19:29).
Ziba had fifteen sons and twenty servants--The mention of his sons and the slaves in his house was to show that Mephibosheth would be honored with an equipage "as one of the king's sons."
Mephibosheth had a young son, whose name was Micah--Whether born before or after his residence in Jerusalem, cannot be ascertained. But through him the name and memory of the excellent Jonathan was preserved (see Ch1 8:34-35; Ch1 9:40-41).