The Adventures of Huckleberry Finn and The Man Who Was Almost a Man are similar because both are about boys who are trying to find their way in a difficult world, mostly alone.
Huck Finn is almost an orphan, because his only family is his abusive drunkard father. Huck wants to be a good person, but he is getting mixed messages on what a good person should be. He sees that people who are supposedly pious lie and cheat, and people who are supposedly bad, like Jim the slave, actually are good people.
But I reckon I got to light out for the territory ahead of the rest, because Aunt Sally she's going to adopt me and sivilize me, and I can't stand it. I been there before. (Chapter 43, p. 194)
At the end of the book, Huck leaves. He does not trust the people, and he wants to make his own way in the world.
Dave is also confused about the world he lives in. He does not like the way he is treated by his parents, coworkers, and boss. He is convinced that having a gun will make him a man. When he buys the gun and accidentally shoots the mule, everyone laughs at him. He is not being treated with respect. His father even tells him he is going to beat him in front of everybody, making him feel more like a kid.
Dave turned and walked slowly. He heard people laughing. Dave glared, his eyes welling with tears. Hot anger bubbled in him. Then he swallowed and stumbled on.
Dave decides to leave, both because he does not want his father to beat him and because he wants to be treated like a man somewhere. He will go somewhere where he can be a man.
Both boys feel that they are not treated right, and cannot trust those around them. Neither feels like they get enough credit. They both feel older than others treat them. This is why, in the end, each boy runs away to live his own life on his own terms.