How do the authors descriptions of the youth beliefs and those of his mother help build tension in the passage

I think you are referring to the beginning of the story. Before joining the army, Henry dreamed of grand battles that "thrilled him with their sweep and fire," and he seems to desire a "Greeklike" or "Homeric" struggle. His mother had discouraged him from joining but he enlists anyway.

When Henry finally leaves, his mother does not try to convince him to be a hero, as he expected. Instead of an impassioned, beautiful scene, his mother gives him some simple advice. She tells him to be careful and not try to beat the entire rebel army himself and not to fall in with a bad group of soldiers. Then she adds: "I don't know what else to tell yeh, Henry, excepting that yeh must never do no shirking, child, on my account. If so be a time comes when yeh have to be kilt or do a mean thing, why, Henry, don't think of anything cept what's right." He leaves his mother, who cries softly as he goes, and joins his comrades on the way to Washington.