How does the relationship between Billy and Boyd seem to deepen as they travel together?

Boyd says he has been waiting for Billy to show up. There appears to be an understanding between them that they will go to Mexico in search of the men who killed their parents. This seems to show a deeper level of their relationship and that relationship continues to grow through the coming weeks as they travel together. Much later, there's a brief discussion between the boys as to whether their father would have agreed to having the boys on this quest. They say they know what their father would have said but don't say what that would have been.