Near the end of the story, we learn that Jim Hickam has gone off to college at Virginia Tech on a sports scholarship. He had always been a talented athlete and this scholarship was the best chance of any of the Hickams to escape a life in the coal mines in Coalwood (at least until Homer's success with rockets). He left for Virginia Tech in 1960, which meant that he was not around for most of his brother's rocket tests in Coalwood. Homer also attended Virginia Tech, and the two would share the long ride to and from the college together.
Jim and Homer always had a contentious relationship in which they constantly teased and belittled each other. In the end, they come to respect each other for achieving success in their respective interests. Jim continues to focus on sports and become a football coach as well as a history teacher, first in Fort Chiswell, Virginia and later at Northside High School in Roanoke. Near the very end of the book, Homer admits "I am now, as I have always been, proud to be Jim Hickam's brother."
Jim had gone off to college on a football scholarship and the last we heard was just absorbing checks to make sure that his wardrobe was keeping up with the fashions. We knew that he didn't really get along with Homer all that well, but that they still cared for each other in a way that brothers will.
At the end of the book we learn that Jim became a very successful high school football coach and that Homer was proud of the fact that he likely served as a mentor for many, many boys as they grew up and dealt with the difficult transitions of high school.