I don't think that Nnaemeka stands defeated in front of Okeke. I think that he is shown to be a bit apprehensive and reticent about angering his father in terms of marriage. Nnaemeka does not openly repudiate his father's wishes. He tries to assuage Okeke and make him accept Nene. This is to no avail, as Okeke is visibly angered and demonstratively upset at his son. Yet, Nnaemeka does not acquiesce and surrender. He lives with Nene and lives his life with her even though Okeke does not officially approve. While Okeke does appear to be more angry and more intense with being more prone to openly showing disdain for his son's choices, I think it's a mistake to presume that this makes him victorious. Achebe seems to conclude in the opposite manner. It is precisely Okeke's stubborn resolve that has taken the best from him at the end of the story. This stubbornness has prevented him from seeing his grandsons and being a grandfather. It is in this where I don't see Nnaemeka stand as defeated. If anything, the ending of the story shows Okeke as fearful and recognizing that he has lost time and might lose more of it if his ways does not change.