Your mindset is that of a big city yankee. You need the mindset of a rural lifestyle where the plantation or farm and the people on it are the only thing you see for long periods. Where your entire work and social world is wrapped up in that rural area.
With that in mind, lets get back to the story or Weary. The yankee mindset pictures all the antebellum South as one huge cotton plantation run by a hands off gentleman planter and worked by thousands of mistreated slaves.
Of course, there were large plantations with hundreds of slaves but in the area of NC where Weary was from the farms were usually much smaller. There's a good chance that Weary's family were the only slaves on this farm and that Frank and his family worked side by side with them in the fields. Without the pressures of the city social scene it is easy to understand that Frank and Weary could have spent many a day fishing, hunting, etc., and not even think about the class system at play.
Given that environment, it's also easy to believe that once Frank joined the Confederate Army, that Weary could have asked permission of Frank's father to join, also.
That's the best way I can explain it.
Thanks for your explanation. I don’t disagree with most of it.
I would think it is at least as likely that Weary was just told to go along. In any case, he had no legal or practical right to make hiw own decision either way, so could be definition not be a real volunteer.