At this point, why does Nick observe, "There was something pathetic in his[Toms] concentration?"its in the first chapter
Nick goes to great lengths to earn our trust as a reliable narrator in Chapter 1, however, these words here show his absolute judgment. Using the adjective 'pathetic', he demonstrates that he will use loaded words to persuade us as readers to make decisions about character.
He spent the previous 3-4 pages telling us about Tom's arrogance about what he owns and how he looks. At the point when this quote is spoken, Tom had been reporting on some of the things he's been reading. Tom strikes me as the dumb jock, and it seems here that Nick is making fun of Tom's strained thought to work through issues encountered in his reading. Tom gave this 'analysis' and Nick could discern that Tom was struggling through an unfulfilling time in his life, and sees him striving for fulfillment through what he reads.
When Nick says this about Tom, Tom is in the middle of a speech about race and the superiority of the white race.
In my opinion, the author is trying to emphasize that Tom is essentially a weak character. He is talking so much about how much better his race is, but at the same time his concentration is less acute than it used to be.
So it seems like he, himself, is sort of losing his abilities or his importance. And it seems like he is using the white supremacy rhetoric to compensate.