That thy master's son may have food to eat, i.e. that he may have wherewith to buy food for all his family, and all manner of provisions (which oft come under the title of food and bread) necessary for himself and them. It is probable he had been the chief steward to manage Saul's lands; whereby he had great opportunities to enrich himself, which also he was very intent and resolved upon, either directly or indirectly, as the following history of him shows; and therefore it is not strange that he was so rich.
Related Commentaries of 2 Samuel 9:10
2 Samuel 9:10
10 Thou therefore, and thy sons, and thy servants, shall till the land for him, and thou shalt bring in the fruits, that thy master's son may have food to eat: but Mephibosheth thy master's son shall eat bread alway at my table. Now Ziba had fifteen sons and twenty servants.