What small lie does the old man tell the boy? Why doesn’t the boy confront Santiago and make him admit he is in dire straits.

Santiago tells the boy he'll have his rice and fish later, and that there is no net to cast. He doesn't want the boy to worry about him.

"There was no cast net and the boy remembered when they had sold it. But they went through this fiction every day. There was no pot of yellow rice and fish and the boy knew this too."