People here rely too much on deleted scenes and novelizations and what not. You can infer a lot about the kid from what's shown, you don't need to rely on deleted scenes.
We know Kirk is running away from home. We know he doesn't get along with his stepfather, who threatened to whip him. We know something his stepfather did made him so angry he stole a valuable antique car. We know it is likely the stepfather called the police on Jim.
We then see Jim wave to another kid hitchhiking on a lonely highway. Obviously, the other kid is fleeing the same situation Jim, for the same reason. The other kid is therefore either Jim's brother or Jim's stepbrother, who likewise was so fed up with the stepfather that he ran away from home. It would strain credulity for two friends to be randomly and independently running away from home at the same time along the same stretch of rural highway for unrelated reasons.
There is no reason to show the scenes with the abusive stepfather because the presence of such scenes can be readily inferred from what we observe. I wish more moviegoers would learn to think for themselves and not need every single plot point be painfully spelled out.
Finally, the cliff scene is critical to the movie's character arc.
