What is the theme of book 16 of The Odyssey, in which Odysseus is reunited with his son?

The abiding theme is one of loyalty. Odysseus, disguised as a beggar, has finally arrived back in Ithaca, his beloved homeland. Eumaeus, the swineherd, doesn't recognize Odysseus, but when he talks to the "beggar" it's clear that he misses his king very much and still harbors great affection and loyalty towards him. This demonstrates to Odysseus that he can rely on Eumaeus in the imminent battle with Penelope's suitors.
Also loyal to Odysseus is his son, Telemachus. Odysseus isn't just his father, but also his king, and Telemachus doesn't hesitate to show his loyalty by agreeing to join him in a potentially dangerous mission at the palace. Telemachus may have matured considerably throughout the course of his epic wanderings, but he's still just a young man and could be forgiven for not wanting to risk his neck against the might of the suitors, but his loyalty as both son and subject ensures that he doesn't hesitate joining Odysseus and Eumaeus in their daring plan to rid the palace of Penelope's troublesome suitors.