Trabb's boy is the local bully and rebel. He's Mr. Trabb the tailor's son, and he likes to make fun of Pip for being too-cool-for-school. In other words, he's just saying what everyone else is thinking. Or, sometimes, acting out what everyone is thinking:
suddenly the knees of Trabb's boy smote together, his hair uprose, his cap fell off, he trembled violently in every limb, staggered out into the road, and crying to the populace, "Hold me! I'm so frightened!" feigned to be in a paroxysm of terror and contrition, occasioned by the dignity of my appearance. As I passed him, his teeth loudly chattered in his head, and with every mark of extreme humiliation, he prostrated himself in the dust. (30.4)
Pip tries to act like he doesn't care, but … he does. If he'd never met the convict, he would have ended up just like this kid, the child of a laborer and pretty happy with his life.