The meaning behind the title (spoilers)
This 1964 film is unlike a lot of the one-sided productions we see in today's culture of victims. It bravely heaps criticism on both sides. Sure, the film exposes whites' racism in all its odious ignominy, and one might thus think that "Nothing But a Man" is meant to describe how we should view Duff Anderson, as opposed to "boy" or "negro" or any other pejorative term used in the film. But the film also casts a harsh light on the black men who abandon their families, abandon each other, abandon even hope itself.
This, rather than the injustice of racism, is the true theme of the film. We see it everywhere: in Duff and Josie's neighbor, Barney, who's lost all hope and just sits around, like "hundreds of men" Duff has known, in Josie's self-serving preacher father who, according to Josie herself, has never done anything to help the Barneys of his community, in the sawmill workers who are afraid to stand together for better treatment, in the blacks who go north, in Duff's absentee father, and in Duff himself who doesn't know his own son any better than his father knows him. This, therefore, is the Duff we come to know in the first half of the film: he's not a son, even though he has a father, he's not a father, even though he has a son, he's avowedly not interested in becoming a husband, he's not a member of any community... he's nothing but a man.
Indeed, the climax of the film isn't in any of the scenes involving racism. It's when Duff's father dies and Duff realizes that he's just as bad at fatherhood as his own father was. Duff has always dealt with life by "running free and easy" (as Josie's father put it) but running away has trapped him in his negative feelings toward whites. It's only by committing to being a husband and a father that he finds a positive force to help him rise above the injustices of the Jim Crow era. And that's why, at the end, he tells Josie he feels "so free inside".
The phrase, "nothing but a man" is a lament on the cycle of family and community disintegration that left blacks weakened in their struggle for integration with whites. Sometimes it's not enough just to be a man.