Sonny is pretty much worried about the reaction of all of his peers when he befriends Quentin. As he notes,
"I didn't have much of a reputation at Big Creek, but it was still better than Quentin's."
Sonny first worries that if the football boys see him shake hands with Quentin, they would accuse him of holding hands with the outcast. Also, as Roy Lee points out, if Dorothy Plunk, the girl he likes, see him hanging out with Quentin, she will most likely "lose interest" in him. Even Roy Lee, who shares a position with Sonny on the fringes of popular high school society, asks him, "What were you talking to that moron Quentin for?" All in all, Sonny is afraid that by befriending Quentin, he will ruin his own tenuous standing in the high school social hierarchy altogether.
Quentin is a loner, and considered to be "the class joke." He uses "a lot of big words" and often speaks with an affected British accent, and he avoids any sort of physical activity by malingering and making excuses. Quentin is also arrogant, and everyone, including Sonny, makes fun of him. Still, Sonny sees him as some sort of genius, and solicits his help because he desperately needs his direction in achieving his goal of making a rocket. Quentin, on his part, agrees to work with Sonny because, although he too has dreamed of making a rocket, he knows that in order to be successful, he must have the cooperation of others. Quentin recognizes that he lacks the leadership skills that Sonny has to get others to work with him. In Quentin's eyes, the partnership will be mutually beneficial, and will enable both boys to participate in the invention of a viable, working rocket.