One possible example of a thesis statement:
In the story "A & P," John Updike presents the growing pains of adolescence. Therefore, the main character displays the typical characteristics of teenage behavior such as rebellion, immaturity, and the need for individuality.
Focus on the theme of "growing up" through three distinct behavioral attitudes. Sammy's attitude (in his first-person storytelling) is very distinct. He comes across as someone who is attempting to tell a story with apathy, but it is clear that he wants people to take him seriously. The main attitude he portrays, to me, is one of a teenager trying very hard to sound like an adult. Therefore, I'm not sure that your thesis should only focus on Sammy.
The thesis suggestion above clarifies your ideas. However, to focus on "rebellion," "immaturity," and the "need for individuality" seems to point as much or even more to the girls who enter the store in their bathing suits. I might tweak it to say something like
In the story "A & P," John Updike sums up the growing pains of adolescence in just one afternoon. Through the choices characters make and the reactions of others to those choices, the author shows that growing up is a mixture of rebellion, immaturity, and the desire for individuality.
This way, you can focus on each of the three attitudes you originally chose, but instead of limiting yourself to Sammy's actions, you can choose from the actions of virtually anyone in the store and even Sammy's family (who reacts to his quitting as "the sad part"). This allows you to stay with your original idea but provides more examples as proof.