At the end of the novel Monster, Steve's father can barely look at him. While his father did show some emotion at the closing of Steve's trial (he hugs him after the not guilty verdict), once Steve is home their relationship changes even more.
Steve states that his father treats him like he does because he (his father) no longer recognizes his son. He cannot fathom that his son would have friends, or even acquaintances, like James King or Bobo Evans. It seems that the fact that his son even was around the murder scene and put on trial makes Steve a stranger to his father.
Essentially, Steve's father acts strangely around Steve because he no longer knows who his son is. Even though he was found not guilty, Steve's identity changed in his father's eyes.