Mental illness is the main theme in "The Red Convertible." Brotherhood is a supporting theme but you could also include more polemical themes like the government's neglect of veterans.
When Henry returns from the war, the experiences he'd gone through had been so traumatic that he couldn't bear to talk about them, even to Lyman with whom he seemed to be very close. Coming back from such an intense experience in which he probably feared for his life on a daily basis (for three years), Henry just didn't know how to function or reintegrate with his old life. Henry was suffering from post-traumatic stress disorder, which during the Vietnam War was not nearly as understood as it is today. In fact, some probably still referred to it as shellshock, the term which began to be used during and following World War I. Post-traumatic stress disorder is a severe anxiety disorder in which symptoms include nightmares, anger, severe anxiety, and sometimes even re-experiencing the original trauma.
Possibly, what made it even more difficult for Henry to function in his old life was that he was reliving those traumatic experiences. Since he received no treatment, his only outlet was to stagnate and zone out to the television. But even that way of numbing himself was not enough to overcome the constant anxiety.