His reaction is one of pain.
In the beginning of this chapter, Jem is cocky (to say the least). He fully believes that Tom will be aquitted. When Reverand Sykes expresses his concern, Jem says,"don't fret, we've won it...Don't see how any jury could convict on what we heard."
However, as the jury comes in, it is clear that things will not go well. The judge polls the jury. "Gulity...guilty...guilty...guilty...". Scout looks at her brother and saw that "his hands were white from gripping the balcony ral, and his shoulders jerked as if each "guilty" was a separate stab between them."
Jem has learned a painful life-lesson. Innocence will not always equal justice.