Sammy is your typical teenager. His audience is definitely someone his own age whom he has no deference for and no preferential treatment in terms of his use of words. He has poor diction, from what one can judge, and his choice of language is quite simplistic and colloquial. He throws in plenty of jargon, and expressions that denote lack of maturity, teenage angst, and a necessity to make his point avidly, the way many teenagers would want to make when faced with a situation of which they have little control. He describes the girl's breasts as "ice cream scoops", and voices his anger against his manager each time he describes anything that has to do with him.
You could certainly state that Updike did a great job characterizing this young man's voice and language with that of a typical teen who is almost whispering his thoughts to us, as if we were one of his peers.